Easy:
1) CS 1.6 Wasted easily an entire year of my life playing this. Bunny hopping on 1.2

good ole times.
2) Goldeneye 007 When it first came out, it was the best FPS of all time.
3) Team Fortress 2 The updates have pretty much ruined this game. They should of stopped after the first 3 updates. This game had amazing balance and replay value. Different classes was invented here and others have tried to copy but you cannot beat the original.
4) Call of Duty 4 Great game. Nuff said.
As far as Crysis 2 is concerned, they should make the HS a little harder to get. It seems any noob can just spray at you and hit you right in your noggin. I do find myself playing it more often though. It is very fun until a hacker joins and starts raping everybody.