PDZ - Online is the only way to play it. Co-op is fun, it's really the only way you should play the story mode. Multiplayer is best with Onslaught, Infection or Eradication, the bots now work pretty well in the Dark Ops modes as well.
CoD2 is great single player, and now the online is great now that the lag is fixed. The Sniper rifle is overpowered, though.
GRAW is also great single player, and the multiplayer is awesome too, except for the watered-down graphics, and no "cover" mode.
Battlefield MC is great for multiplayer if you like something along the lines of Big Team Battle in Halo 2. The physics with vehicles are annoying (especially tanks), but they finally fixed the voice chat.
Hitman Blood Money is also fun, but if you didn't like the previous ones it won't change your mind.