You could sit here and argue and kill each other all day and noone will ever be able to convince the others of his/her opinion. But whatever, I might as well try.
World of Warcraft
Sets a whoping amount of records and is esitmated to make more than $300 000 000 per month for subscriptions alone.
Counter Strike
I never liked it that much but I have to give it props for the sheer amount of people playaying it, tournaments hosting it, and hours spend on it.
StarCraft/WarCraft III
Probably the most succesfull RTS in existance. Perosnally I think they're better than the Age of * games and the C&C serries (their prime rivals)
F.E.A.R.
Call me crazy (I probably am but I liked this better than Half Life 2, Doom 3, Quake 4, and all them other crazy shananigans.
Rome: Total War
Just because :P
So there. Best game(s) >_>