I'm gonna lay down the shit on the first 3 splinter cells here so listen up faggots
Make sure you get the console versions, the PC versions don't have official controller support, which you N E E D if you're going to be playing these games, it's just not gonna work with a mouse and keyboard. Also I'm going to be talking about the Xbox versions here since that's what I had.
First game is fantastic, played the shit out of it when I was a wee bab, absolute blast to play. Looks great, controls are good, sneeki breeki is very well done, but laughably stupid at times (you can face a wall in a dimly lit hallway and guards will walk right past you, it's laughably generous with keeping you hidden) Rarely does the game feel unfair, but there are a few parts that are really annoying (Spoilers: freezer section, reading thermal prints on keypads in less than half a second or mission failed), and there are some parts where you're forced into open combat, but those are surprisingly well done (Spoiler: The farmhouse shootout is one of my favorite sections in any game ever just because of how many ways you can kill the guards, frustrating as hell but so much fun) and aren't really unfair. Controls and AI are a bit janky wall mines, but a great and cheap game to pick up. Music is pretty good too.
Pandora Tomorrow isn't too much different from the first one, but that's a good thing. The few things it does change makes it slightly better than the first one. Gunplay is fantastic and so satisfying to pick off enemies with. Has more of a MGS feel to it than the first one, looks a bit better with more colorful and unique environments. Some sections are a bit tedious, but again, rarely does it feel unfair. Highly suggest.
Chaos Theory I haven't actually fully played through, but it's a lot different from the others. Firstly, it is hands down the best looking game I've ever played on the Xbox, and it keeps a steady framerate almost all the way through. With that being said, they changed a LOT from the previous two entries. Side missions, less linear levels, different control scheme (actually the control scheme between all three games is vastly different, kind of annoying to relearn everything), new gameplay and environment mechanics, everything got overhauled. I'm still not sure if I like it more than PT though with everything it changed. But the base game is still there, and is fairly easy to pick up on.
As for the other entries, I haven't played any of them, but they apparently dropped off a fucking cliff in terms of quality after Chaos Theory.
TLDR First two are fantastic, third one is good as well but different.