I want to highlight two mod makers, instead of single mods. lStewieAl and WallSoGB for their efforts unfucking New Vegas. Being forced to pick a single effort, the engine optimizations is probably the most impressive. Honorable mention to all the various script extenders that make so many fantastic mods possible in the first place.
Splinter Cell was primarily an Xbox franchise, so if you’ve got an Xbox of any kind that’s probably your best bet, assuming they’re still for sale. I believe they were all developed for Xbox and ported to other platforms. For Chaos Theory in specific though the PC port was pretty good, so if you don’t it’s not a big problem. Possibly some controller issues that need a fan fix? Though on the plus side you can find a wide-screen fix mod for modern resolutions. The first two games had worse ports, if I remember right. And the Playstation and GameCube ports were pretty universally bad I think, for all the games.
The one where the console version is really important is Double Agent, but that is a whole other can of complicated worms, because then it wasn’t just the matter of bad ports - they released two completely different games with different mechanics, levels and story executions (albeit the same overall plot).