I don't think I could really pick one as my favourite, as they all have their own pros and cons.
The campaign on COD4 is fantastic, but when I first played it on the PS3 I didn't play the MP.
World at War had a great campaign, but it was slightly marred by the ridiculous grenade spam on Veteran. The online play was great - especially the War game mode (which I hope returns in Black Ops), but having played the Ricochet game mode, and with MW2's host migration, playing it again seemed a bit of a backwards step.
Modern Warfare 2 is a better all-round game, with a decent campaign, fun Spec-Ops, and addictive multiplayer.
Having said that, I'm really expecting Black Ops to nail it. If they can take the best bits of World at War and Modern Warfare 2, they'll have one hell of a game. Add to that the Wager matches, Combat training, Create-a-Class 2.0, Co-Op, Theater mode, and the new points system, and I think Black Ops has the potential to be the best COD game to date.