Addition of knife = The greatest stealth game ever made
What can I say, I've got a hardon for this particular brand of realistic gameplay. Also why I find Counter-Strike such a disappointing and disgusting phenomenon. Ah well, back to Sam Fisher. Splinter Cell was one of the greats, and Chaos Theory, the third one in the series, just takes everything from Splinter Cell and makes it better. Better weapon mechanics, better graphics, better lighting, better enemy AI, better level design, and holy shit they added a knife for Sam! Truly, the most important addition to any series, ever. Slicing someone's throat open from the side with it is one of the most satisfying moves in gaming.
Chaos Theory is also superior in its addition of a fantastic co-op mode online. Instead of sneaking around by yourself, you could actually sneak around and perform stealth knockouts with a teammate. While limited to four levels, it was not a thoughtless addition and provided some genuinely exciting and tense action. While not anywhere near as cinematic or annoyingly labored with its storytelling as the Metal Gear Series, Splinter Cell is the superior stealth genre series in my humble opinion. PS: don't get SC Double Agent, it's a buggy mess on the PC.