Multiplayer: Black Ops 
Single Player: Modern Warfare 2 (didn't finish CoD4 and didn't play WaW) 
Overall: Black Ops

By "modern" I mean after they added an experience system / unlocks to multiplayer, not modern in the sense of when each game was supposed to have taken place.

There is only one answer to this thread.  
 
I don't even need to say it.

You sure do make a lot of threads...

I lol'd at all of the suckers who said Black Ops and Modern Warfare 2.

COD4 for both its single player and its multiplayer. Its single player is filled with so many great moments and its mutilplayer is basically the one I spent the most time with.

COD4 Modern Warfare.

W@W motherfuckers, because I wanna be different.  I didn't play much Cod4 because I was to busy playing Halo.  

Black Ops. Finally a Call of Duty game where the protagonist wasn't a floating camera.

I really really enjoyed mw2, but it was pretty unbalanced imo. CoD4 also had imbalances but they were much more tolerable!

World at War had Kiefer Sutherland's voice. Fuck everything else.

E: And Gary fucking Oldman.

Blops. Best story by far, and I loved the Cold War setting.  
 
Also, Gary Oldman. And Ice Cube. 

i think i had the best time with Black Ops.  i never got a chance to play the first Modern Warfare game and Modern Warfare 2 was a fun experience but i was hooked on zombies at that time so i kept going back to World at War