KZ2 by miles...
Addictive, strategic multiplayer - Squad based, thoroughly enjoyable, and I love the class system
Genre defining graphics
Feels like you have weight, not like a floating gun
Cooler enemies
Am I the only one who loved the cover system
Single player was massive fun, especially the final level on Elite Difficulty.
Mostly its because I love the multiplayer sooooo much...I just love this game