Battlefield 3. From my experience, there's never a dull moment! Best multiplayer I've played since cod4.
Crysis 2 is good, but drags on a bit.
Killzone 3 has a good single-player, good multiplayer and it should be petty cheap by now.
You're better off buying map packs for MW2 or Black ops if you wanna play cod.
The rest, I dunno.