Still the best game in the serie.
It was made when AAA stealth games were actually challenging, and punished you hard for going full rambo.
The plot isn't really memorable, it's your average action movie scenario involving really bad guys and your witty american hero saving the day. Nonetheless, Sam Fisher is a character you'll find easy to like, and can make you crack a smile.
When it comes to gameplay, it's kinda frightnening to notice how rigid it is, how the game got old. It gives you everything you would expect for a good stealth game, a lot of possibilities, some alternate routes, and a heavy dose of "waiting in the shadows hoping for that guy not to see you".
Graphics-wise, it's holding up. When the game came out, it was one of the most beautiful thing I had laid my eyes on, it really blew me. It was on par with all the AAA games that came out at this time, maybe better.
Soundtrack was composed by Amon Tobin. Yep, Amon Tobin. As a result, Chaos Theory has an atmosphere no other Splinter Cell game has, and god, they wish they had it. It's a real pleasure to lurk in the dark, your knife ready to stab Generic soldier#54 with a stressful, sometimes a bit creepy, note from the wonderful soundtrack.
And co-op has to be the best co-op experience I ever had on any game. It's challenging, it requires communication, and you can climb inside a vent by touching your partner in places you wouldn't even have thought about in the first place.