Counter Strike Source
Left 4 Dead
Half Life
Call of Duty 2
TES4: Oblivion
edit: Just noticed, I <3 Valve
I can see games such as Fallout 3 going on that list if and when good mods come out. Also games like World Of Warcrack would be 6th, GTA4 would be on my top 10.
Halo was great for it's time and still can be fun. In my opinion halo 2 wasn't nearly as good and i haven't played Halo 3 enough to comment on it.
I liked the open levels on Crysis, having the option to use stealth to take out your enemies, and the only game with better graphics than Cryisis is Warhead.