After COD 4:
Call of Duty World At War - Pretty good campaign and very fun zombies. Multiplayer didn't stick with me at all.
Modern Warfare 2 - Piece of crap, avoid at all costs
Call of Duty Black Ops - An actual good campaign for a COD game. Zombies is fun. Multiplayer is boring and quickscoping is still possible.
Modern Warfare 3 (Nov. 8 2011) - I'm renting/borrowing to finish campaign/achievos on 360.
WaW - Horrible. Terrible multiplayer and single player got boring really quickly.
MW2 - My favorite CoD game so far. Very fun multiplayer which keeps you pretty interested, but lack of players on PC. Lots of players on PC and 360. Nice single player too, you could play it a couple times through and not get bored.
BO - Did not like the game at all. Multiplayer is a joke. Multiplayer weapons are horribly unbalanced, and 90% of the people in this game use the most overpowered weapon, which is the FAMAS. High rate of fire, extreme accuracy, great range, unbelievably high damage. Single player sucked so bad. I was really bored trying to beat the single player of this game. The only reason I ever play BO anymore is for Wager Matches, which are the only fun thing in the game IMO.