Are these games fun on their own or do you need mods to get enjoyment out of them?

Depends on what you think is fun really.
The only mods I've used for the first two are Reclamation Packs, which are like patches pretty much, and Absolute Nature and Buildings mods, which are old graphics mods.

I think some extra UI related stuff too, but nothing game changing.

I've yet to play Call of Pripyat.

They are very atmospheric games, fire fights can be intense though. I enjoyed the first one more than the second so far.

TBH I havent used any mods for any of them. I think the vanilla games, as are, are well done. Although I am going to look into some mods now that I have finished both SoT and CoP

No, S.T.A.L.K.E.R.: Shadow of Chernobyl was buggy as hell wen it was released in 2007, but it was patched over time by the original developers. Unfortunately, its state of release has caused a stigma associated with the game.

The original game as it is, is fine. But it's highly recommended you also apply the Zone Reclamation Project, an ongoing community bug fix patch, it's one of the oldest "mods" still being developed and the best one to add.

I also recommend the Zone Reclamation Project. It fixes many annoying bugs and allows customization of the game.

The graphical enhancements aren't necessary, but improve the visuals and atmosphere of the game by a lot. I'd install Atmosfear 3, Absolute Nature and Absolute Structures.

You might also wanna look at Original Weapons Renewal, which improves the original weapons without adding a million other weapons and items.

But this is kind of what I'm talking about. Already people are recommending a lot of mods. But about the vanilla game? Does it stand on it's own?

I have not played the vanilla SOC since ca. 2008 and never completed COP without mods, so I'm not sure.
I would at least use the ZRP, as that is basically a patch, not a mod.
Best approach is probably to load up the game and see how you like it. If you feel like something is not right, ask around what mod might offer the improvement you are looking for. You can complete SOC and COP without mods, though some quests will likely glitch out. Clear Sky vanilla I would not recommend.

Call of pripyat not really, the game is pretty enjoyable without mods too. Though i've never used mods on any stalker games or most of video games i played. Call of pripyat is the most polished game of all stalker games, for some reason i enjoyed call of pripyat the most. One mission did glitch out since i couldn't get the papers from some guy into my inventory which was somehow important for mission or ending but that was a long time ago i played it, still fnished the game.

Shadow of chernobyl i really don't know anymore since i played it back in the day, would feel like a pretty limited experience since it's missing those call of pripyat features and it probably has some bugs somewhere though not sure.
It's also crazy when someone managed to kill a merchant in SOC being inside you can't shoot but someone threw a grenade or don't know what.Also SOC is a pretty unique game good on it's own if you know what you need to do, but don't know about the mod part, did enjoy it still.
Clear sky is not preferable for the vanilla game finale pretty much glitched, but more to my luck since i was using horrible armor scientist suit or something and that was pretty bad for the finale how many times i got killed because of that.
Also how they made bloodsuckers hittable here is devastating bad.

If you are not the incurable purist, you absolutely must at least have a try at the ABR Mod for Stalker COP which was released only last year (!).
Why? Because you will regret it till the end of time if you dont. Trust me on this. No other Mods required.

The only excuse not to do this which I could accept is when your Rig and Graphics Card is still from 2009 as is the vanilla game.

Wht's the abr MOD? I did try a few mods bust I didn't like some of them. Complete mod made the game too easy.

The ABR mod contains mostly graphical updates and bug fixes. There are also some new weapons, but no gameplay changes as such.

https://www.moddb.com/mods/abr-mod

Look for the Lost Alpha on Google. It's got some extra stuff.

The Stalker: Lost Alpha Director's Cut is great. Basically a mod team researched and recreated the original vision for Shadow of Chernobyl that the Stalker Developers weren't able to implement. Going from SOC to this is like a completely new game at times. Other than that I play the Call of Chernobyl mod, which is basically all the games world spaces and features combined together into a open world sandbox. With a few good addons and some creativity, Call of Chernobyl is like the Fallout/Skyrim/Dayz free-roam sandbox you never knew you needed.

Very late to the party as I always am. Some people will likely not agree with me on this, but Shadow straight outta the box had me hooked for some of the longest sessions I've ever pulled and to this day haven't patched or modded it, but I came to view the bugs as a reminder that you're ment to always be at a disadvantage at the worst possible time. CS was a mess so I only played thru once then waited for COP, which is great just short, played it with just the patches. Been in Call of Chernobyl for awhile now and you can take it any direction you want. COC is where modding really shines in my opinion, but there are good ones for every game in the series. Try to roll thru em once with just bug fixes and quality improvements at most before getting into the mods cuz you likely won't go back after