Coincidentally, I've only played the first CoD game. Therefore, I have to go with the first one. Yes the original Call of Duty. Honestly, the entire series is good.
My favorite Call of Duty would have to be Call of Duty: Finest Hour. There was just something about that came that always kept me interested. I'm a huge fan of the whole CoD series though, but that one just stands out to me.
I would have to say as far as Campaign goes, Black Ops had an amazing story line. Can't beat Vietnam! That was fun with the swimming underwater, apache's, pontoon boats. Fun stuff!
I did like MW2, but the thing that ruined it for me was the "Noob Tubes"... Everyone would just run around, hit each other with a grenade launcher, and run off again.
im gunna saw mw2 for the fact i only played that and the first cod which wasnt that great except story mode was decent but for now mw2 untuil i play black ops at christmas