I was more into Mario, but thought I would give Sonic a try. Sonic Adventure DX (PC) seems nice, although I'm not too sure what to feel about the 3D. Sonic 3 & Knuckles (PC) on the other hand seems like the best 2D iteration of the series... Any advice? I prefer my games challenging, if that helps any. ;)

if you play sonic start from the first one then go to 2 trust me you will like 3 and S&K a whole lot more. also if you get sonic and knuckles get 3 with it they go together and make a full game sonic 3 is part 1 and sonic and knuckles is part 2. its pretty much a 2 disk{cartridge} game :P and all of them are challenging and very fun and as you play threw 1, 2, then 3/S&K you can really see how the genesis hardware was used and how it got better over the years :)

sonic adventure is also a great game it has alittle bit of everything there is speed runs with sonic or fishing with big the cat there are multiple story lines with each character so you see the story from different perspectives and if you loooove a challenge and are a competitionist / perfectionist there are awards to get and when you unlock all of them {goodluck} you unlock another character :D anything before sonic adventure 2 is a great sonic game and if you can get the handhelds {rush and such} those are amazing as well

but in all honesty i think you should start with 1 if you have any questions just ask and i will help out best i can ^.^

Just get the Sonic Mega collection + ,on PS2 or PC

Do what Roman5 said, and then follow what SladeStrife said.

Sonic 3 & Knuckles is awesome, but you should definitely build up to it, because the games before it are also worth your time and might be lowered a peg if you play more recent games first.

I am also a huge Sonic Adventure fan, all emblems on both games, etc, but there are a lot of people who think the games are complete garbage. All I'm saying is whether you will enjoy them or not is up in the air, but if you can enjoy them they are excellent.

EDIT: And Sonic Rush is fucking great. The music is reminiscent of Jet Grind Radio due to the guy doing the soundtrack, as far as I know. If you've played Jet Grind Radio you'd probably immediately consider Rush a must-have based on this alone, but the game itself is also pretty great.

I'd definitely play up to it. Personally, I dump my own carts, mainly because I played them on Genesis and I prefer the authentic look and feel.

Probably the only thing that I didn't like about the original games was the lack of saves.

Play up to S&K, then replay the versions that S&K mods out with new characters.

Sonic 3 & Knuckles is definitely the highlight of the Sonic franchise. I personally enjoy Sonic and the Secret Rings, but that game does have some control issues. If you want to try out Sonic in a modified DOOM engine, Sonic Roboblast 2 is worth checking out. I prefer 2D platformers over 3D ones, but that doesn't change SRB2's merits.

Sonic 3 & Knuckles (NG+ TAS gameplay video)
http://www.youtube.com/watch?v=rudMhYp4ZS0&feature=related

Sonic Roboblast 2 (gameplay video)
http://www.youtube.com/user/SRB2SonicTeamJr#p/u/6/3ZcW-JSZgYQ

Sonic Roboblast 2 (Homepage)
http://www.srb2.org/

Oh hell yeah

Both Rush and Rush adventure on the DS are great games but in the first game one of the biggest highlights is the music, it was composed by the guy who made the music for Jet set radio, so naturally - it's amazing

http://www.youtube.com/watch?v=sx0CQDc2rMU&feature=related

I wish this guy would do music for all sonic games

I second that. It's a great compilation, with nice extras (Comix Zone, The Ooze, some scans of Sonic magazines).
Stay far away from new gen crap, the Sonic's you want to play first are 1 to 3 (Genesis), and Adventure 1-2, I think (<have heard good things about it, but haven't played yet)! Unless you want to have a laugh, then go play Sonic the Fighters or Sonic Unleashed (werehog durrr).
SEGA fucked with Sonic too many times, but there are some decent Sonic titles. For example, if your looking for something more recent, Sonic 4 episode 1 is a good start. I've heard good things about Sonic Colors and the next Sonic game will bring him back, so I hope they finally learned something from all the mistakes in the past.

Now that I think about it...the best course of action for you After playing the Mega Collection + is getting Sonic generations if you are new to the franchise

The game will be amazing and contain the best levels from most games

Get it on consoles or PC

One thing I would caution you about Sonic Adventure, though, is the camera. Much as I love the game, the camera can be more than a little annoying at times.

Also, Sonic can be a little hard to control at high speeds.

But aside from that, as everyone else has said: start with Sonic 1 on the Megadrive, and work forward from there.

GamersGate got a Sonic sale. Are Mega Drive Classics emulators any good?

Also, does GG version of SA DX DC come with bonus GameGear games?

I see... Thanks for the help guys! I think I might wait for the upcoming Sonic CD remake. :)

It's been a while since I don't play a sonic game, but I'll be buying Sonic Generations for the 3DS because it looks great.