Ubuntu, as I really value the time spent on polish, and making certain that every part of it works together with every other part.
Of course, I've drifted quite far from stock Ubuntu over the years, so at this point what's actually mostly keeping me on Ubuntu are just its LTS releases. I hate it when things change for no good reason, so only needing to upgrade my OS every four years, and then spending a day or two tweaking it until I'm happy with it, is the closest thing to the theoretical nirvana of never ever having to deal with this at all. Personal experience: My first several attempts at using Linux failed because of dual boot. I'd boot into Windows for something, and then just stick around in there since rebooting would be too much of a barrier. Only when I installed Linux as the only OS on a system, did I stick with it, and realize I never felt any need to return to Windows. Yes, with qualifications. The majority of Windows-only games Just Work with wine out of the box, nothing else needed. The remainder is frustrating - most of them you can get to work anyway, with various winetricks, but more native Linux games there are, the less often I feel like running the risk of needing to do all the research into which arcane combination of winetricks and configuration changes are needed this time. So unless I really want to play a Windows game, and have done the research beforehand that it's possible to get it to work with wine, I tend to not bother much these days. This used to be true, but I haven't personally encountered any issues in a long time, and these days only to do the most cursory searches for issues before deciding upon new hardware.
Specifically for GPUs, it might help that I stick to the open source AMD drivers.

One of the first decisions that you have to make when you install Gentoo is what profile to use, and knowing what systemd is and what it does can be helpful to decide whether to use the systemd profile or just stick with the openrc one.

By the way, you can use debian with a different init system; to use plain old sysvinit, just install the sysvinit-core system. (And you can use OpenRC, or at least are intended to be able to use it, by additionally installing the openrc package. (Note that installing sysvinit-core will remove systemd-sysv; this is normal, and re-installing systemd-sysv will switch you back to systemd.)

OK. I haven't installed Gentoo from scratch in 15 years, so I have no idea how that works any more. I wonder, but not enough to actually look into it, whether or not they've ever improved the process or at least the documentation in that regard. In any case, it's a reversible decision, so I don't think it's worth fretting over before even installing the OS. I'm sure most casual users will be turned off before it becomes an issue, anyway. It's one thing to be able to select your init system. It's an entirely different thing to be able to actually use it. Debian's decision, as far as I recall, was to not put any effort into providing even sysv init scripts to replace systemd config files. Systemd also insists on taking over a number of other tasks, which need adequate replacements (e.g. eudevd and elogiind; I don't know if there's something for the logging, since e.g. bluez doesn't seem to log anything w/o systemd) which would need to be pulled in instead of direct dependencies on systemd, like Gnome used to have (not that I use gnome, but some apps pull in gnome on the side, since there's no USE=-gnome in Debian). That's why Devuan popped up: to take care of those issues. Not that I really care any more, since switching to Debian would take a lot more than just keeping that crap off of my system.

• What Linux distro you use the most and why?

Mostly Linux Mint, I do change it up every once in a while.

• If you use Linux alongside Windows or other OSs, do you use Virtual Boxes or Dual Boot? Why?\\

A few computers are dual boot. Some are single OS. I would like to get GEOS running as a main OS.

• What are the advantages and disadvantages of using two OSs in the same PC?

Advantage is easily switching between the two. I really don't feel there is a disadvantage for me. I have a ton of hard drive space so I really don't care about space being used.

• What are the advantages and disadvantages of using ONLY Linux (any distro / comparison of distros, if you want to explain)?

Customization is an advantage for me. It pushed me to use more FOSS. The disadvantage is that a bunch of games I have require some work to run.

• Does WINE really works well with Windows exclusive games?

I've had mixed results on the unsupported games. The fully functional games work. There is a ton of games that have Linux installers.

• Is it true that Linux have compatibility issues with some hardware (GPU, specially)? What's your experience with this?

Years ago there was bad linux support for Nvidia cards. Pretty sure that isn't the case now. Never really had issues back then. I used ATI and Nvidia. I had more issues with WIFI cards in laptops. I'm pretty sure that has changed. I really don't use WIFI for any of my Linux laptops.

What Linux distro you use the most and why?
Debian because I like it, it seems to work well and it has a strong focus on free software.

If you use Linux alongside Windows or other OSs, do you use Virtual Boxes or Dual Boot? Why?
Dual booting sounded like a faff to set up with Windows so I've never tried. I prefer the VM method so I can access both systems at the same time. I use a VM with Linux in to test things that I don't want on my main system. For Windows I just use an old laptop that had it preinstalled.

What are the advantages and disadvantages of using two OSs in the same PC?
Disadvantages of dual booting: I often forget to choose the OS before it automatically boots to the first choice so I have to reboot. Also, I tend to only go on my main OS and any others turn into neglected, wasted space.
Advantage: You only need one machine.
Advantages of VM: You can access multiple systems at once, they are easy to just delete and you can clone them.
Disadvantages: Tthey take up a lot of resources so will probably run slower than dual booting and it can be tricky (or impossible) to use your GPU in the VM.

What are the advantages and disadvantages of using ONLY Linux?
Advantages: Less (or no) data harvesting. It doesn't cost any money and you can reinstall as much as you like without having to activate anything. You only have to remember how to do things one way (same as if you only use any other OS).
Disadvantages: Some programs I like don't run on Linux (but there is Wine to almost fix that). Some hardware is more tricky to get working.

Does WINE really works well with Windows exclusive games?
Some of them work really well, others work pretty badly!

Is it true that Linux have compatibility issues with some hardware (GPU, specially)? What's your experience with this?
Yes. On my PC it booted to a black screen when I first installed Linux so I had to figure out how to install the GPU driver on the command line. Some distros would include those non-free drivers by default but Debian doesn't.

Windows Subsystem for Linux 1 and 2, are they really usefull for learning Linux in a Windows environment? Do they really have compatibility issues with some more advanced Linux commands?
Sorry I can't answer this one since I left Windows before they started offering it. On the other hand, OP could watch out for the anti-Linux fanatics.

Thanks everyone for the replies, really. I understand your point, but FOSS is much more than "Free Open Source Software". The concept of FOSS is about "Freedom of Speech", which means, Freedom of software usage, sharing and refinement through community experimentation and expression. Even if Windows have a larger library of "free" apps and tools, they are not "FOSS" in concept. Commonly, they come with ads, bloatware, spyware, etc for "data harvesting" - so they can "continue being free". The price you pay to use them is information and usage data.

Thank you for your answers and experience with Manjaro! Thank you for the general explanation on Linux, WINE, Debian and Arch.

I started using Mint through this research. It's really like windows (GUI-wise). They also have a huge library of documentation in my country language, which is nice!

I may stick with the basics for now as Arch will probably look like programming in assembly for a beginner. haha
As for GUI, I may be weird, but i find CLI commands, smoother, faster and much better than any GUI.
That's one of the reasons, together with FOSS mentality, that is pushing me to use Linux - develop CLI skills. Thank you for your explanation on WINE, that's really nice. I love shell/terminal/cli commands, as said before.
I will need to study this. Thankfully, documentation is huge. :) I will search for this, thanks. Never heard of Nixos. The learning curve, as you said, may be a reason. Thank you for the recommendations and explanation on notebook cards and Linux From Scratch! For now I may keep testing Mint, maybe install Debian and/or Fedora later. But I don't really care about 'new software'. I understand that old software (talking about FOSS here) tend to be more stable and usable as the community improve many tools, when they're not abandoned. Thank you for your reply and explanations. :) Thank you for your detailed explanation! I read it all.
I might stick with your mentality, as Im not really into cloud stuff too. The more my pc is disconnected from the internet with full capability, the better. The reason many of us got away from Windows is to free ourselves from the dependency. Why I'd want a Windows environment in Linux? Better stick with Windows, imho. But I understand Ubuntu users too. Some people need this. Also, thank you for the AMD explanation. The Intel and AMD recommendations might really come in handy, I've never heard of that before. I haven't tested WSL1&2 yet, but Mint in VM worked fine. As for modern games, I don't really care much. Probably I will read Wine compatibility list later, if they run the classic games I have, Im fine. :) Thanks again.

Thank you for your quote. I believe it may be a process to get used to how a new system works. Thank you for your explanation on hardware and One Boot,Dual Boot/VMs. :)

Thank you for your quote and time. User control is something I want . Kinda tired of windows updating and messing with overall settings.

Thank you for your explanations on Dual Boot, VMs and WLS, really.
I noticed that VMs affects system performance, so gaming through VM might not be an option for me too.
Thank you for your wishes too. :)

Thank you for your answers and personal experiences too.
I don't really care for AAA recent games, since I play mostly 25~5 year old games. As I said before, community experimentation through time is something I also value, so WINE might be precisely what I want for gaming in Linux. Looks like I already fell in love with it too. :) (Just need to keep studying it.. haha) Thank you for your wishes too.

Thank you for your answers.
Many answers are pointing out compatibility issues with Wifi adapters. It might sound strange, but that's good in my case imo. I only use wired connection anyway, so even if there are problems with this, It's fine.

Now, about the AMD drivers, Im really surprised as I had no idea about this. Just heard that people had issues with GPUs+Linux in the past. Thank you again.

By an offline repository you mean, for example, a Github repository with sources saved offline? May you elaborate in that please? This looks really interesting.
Also, thank you for the list and your personal experience.

Thank you for your detailed answers with your experience, really. That really helps a lot.
Talking about compatibility, Im really hopefull that the community will keep the FOSS scenario growing for the next years. World 'freedom' is getting pushed to the limit, so people will find ways to fight it in all areas, including software.
Thanks again. :)

Thank you for your alerts and explanations about compatibility too. In reality things require much more work to make things, well, work. Im really thankfull for your explanation about drivers, this may prepare me for the 'fight' Im getting in.
Thanks. :)

Haha, I understand what you said. Yes, comparing all the answers I know what might be best for me to do to stick with Linux, instead of 'giving up and sticking with Windows because - lazyness'.
Thank you for your experience with Ubuntu. I share your -hate for things chaging for no reason / without user control-.

Thank you for your answers. Let's hope Linux gaming and FOSS scenarios keeps growing.

Thank you for your experience with Debian, Dual Boot, WINE and for your answers overall.

- What Linux distro you use the most and why?
Arch.

I distrohopped a lot in the beginning, starting from the apparently beginner friendly Ubuntu flavors, then Manjaro with different desktop environments, and then finally had the courage to try out Arch and settled on it as my one and only distro.

There didn't seem to be many Arch users around here. Arch is supposedly more 'unstable' or breaks often compared to the LTS Ubuntus or flavours, but I disagree. Having access to the latest fixes, patches, versions makes up for any possible breakage. EVen if an update messes up something, it gets fixed remarkably quickly usually. So bleeding edge rolling release distros are for me - I think I had Linux Mint for a whopping 2 hours before I uninstalled it in frustration. I can't stand watching new features being implemented and not having access to it in a relatively short amount of time. I wouldn't recommend Arch as a first distro to anyone though.

• If you use Linux alongside Windows or other OSs, do you use Virtual Boxes or Dual Boot? Why?
• What are the advantages and disadvantages of using two OSs in the same PC?

Nope. Linux is better, Win10 was redundantly taking up space on my hard drive when I 'dual booted' and never used it for 10 months. That's when I uninstalled it forever.

• What are the advantages and disadvantages of using ONLY Linux (any distro / comparison of distros, if you want to explain)?
Advantages: Oh dear, not again :) Echoing many others, it's the usual reasons why Linux>Windows for most people. I can't be asked to list them all, but [insert here bulletpoints of pros of Linux vs Windows] are good examples.

Disadvantages: Some software just isn't available and doesn't work in WINE. If this software is important, and you can't get parity with Windows, then that's a huge disadvantage. MS Office is such software, but even MS Teams whilst having a Linux version, is missing features so parity is not achieved again. Thus, I am stuck with Windows on my work laptop. It can't be helped. I don't see MS Office ever working on Linux - I'm pretty sure it's done on purpose by Microsoft.

Also some games might not work great on Linux or at all, like kernel level anti-cheat multiplayer games. I could care less, but I'm not everybody.

• Does WINE really works well with Windows exclusive games?
Amazingly so. And with DXVK, the performance is awesome. The only thing is (apart from those anti-cheat games), DX12 games on nvidia 10xx cards - there's a big performance hit and vkd3d-proton (the DX12 to Vulkan translation layer) doesn't currently help with those cards (which I have, unfortunately). But this is quite specific to my hardware.

• Is it true that Linux have compatibility issues with some hardware (GPU, specially)? What's your experience with this?
You're clearly referring to Nvidia due to their proprietary drivers and the common belief Nvidia cards have a lot of problems on Linux. THis is quite false, in my experience. Once you get the hang of things and know how to update GPU drivers and kernels without breaking things, it's smooth. Overclocking, custom fan and power profiles, monitoring, all is fine on Linux with Nvidia. I have a desktop 1070gtx, and a laptop 960M optimus hybrid setup, and I can run every single game I've played in the past year. That means 100% of games I have played or attempted to play, worked (almost all with good performance).

And to go back to hardware - I have a soundcard that's a bit exotic, with a headphone amp builtin and quite good quality. It's about 10 years old by now I think, so Win10 drivers do not exist for it. And even finding the win7 drivers is a massive pain in the arse, you end up going to dodgy sites and forums and doing all sorts of registry edits and tweaks to even get the sound card working. Linux? Drivers part of the kernel, detected easily during bootup. Configurable and controllable in Linux, can set any headphone amplification according to your impedance level. Mind blown. Just an example of how, for me, Linux is superior in many cases.

Haha, thanks for your answers too. Somehow I understand what you're saying about frustation on Mint. Maybe you're more used to pure CLI and complexity of Arch? I don't know how complex Arch really is, but it seems waay too much for me right now.

By the way, just finished installing Linux Mint in a Dual Boot with Win 10.
Im really surprised by it's compatibility. No need for any driver install manually. Just USB Booted it, tested the lite version, installed, done. Im using it right now. It's really fast compared to Win 10 imho. Way faster than testing it in Virtual Box.

Thanks everyone for the motivation, tips, alerts, etc.
Im really used to the Terminal already with basic commands (Thanks Termux in Android for this too.) and it's really fun to explore how this system works.

--edit:

I was reading somewhere recently that Mint isn't supporting "Snap" while other Distros are pushing it to the users, or something like that, pardon my ignorance, I couldn't understand what they were talking about.

'Snap' is reffering to the Snapstore? If yes, what's the problem with it that Mint devs won't accept it?

Basically Snap doesn't quantify enough of the components as open source in order to be included under most packaging specifications. Debian itself has a rather pure approach to the standard of free code.

Also, it being associated with CANONICAL tends to be a real poisoned well moment.

It's one of Linux software distribution systems. Actually, you can install snap packages on Mint.

Snap applications run in sandbox instead of having direct access to the system. So, you might experience low performance and other possible issues.

Snap packages contain all their dependencies. Therefore, you will download and store a lot of copies of the same dependencies.

I don't recommend using snap, unless you really need it's features.

Can relate. I was amazed to see some old SoundBlaster Audigy (?) fully supported with kernel drivers out of the box on Mint, that card is roughly 20 years old.

Most of the hardware compatibility I've seen are due weird-ish parts. My old tablet had a wifi problem but on next Mint version that got corrected. The Microsoft Surface line is pretty much compatible but only recently some Surface 3 (2015) drivers and fixes got in the kernel (including brightness control and touscreen).
I can't recall a single driver incompatibility with desktops though, other than swapping the old Mint hard drive between diferent computers, were the installed nVidia driver would crash on any computer with AMD Graphic card. Simply purging the driver fixed the problem.

As you're probably aware in Linux you install software through packages, which are normally hosted on servers. I wanted a distro to be able to access those packages at will even without an internet connection. With Debian you can download those packages in separate ISO's and build up an offline repository.

I don't know which distros offer this besides Debian, but for my use case it's been helpful. The only downside is that it takes some storage space (100+ GB).

100gb+ omg:O why do you want all those packages?

Hmmm would be better if the iso's are extracted and use my md5-identical-file finder and remove duplicates, building the ISO you want at a given time rather than huge duplicates, especially since there's bound to be huge packages (Xwindows, GNU, GCC, etc).