Either WaW (my personal favorite) or MW2. I loved WaW because I was really good at it, simple as that. Plus it also has the best maps of any CoD game. However, I'd say MW2 was the most fun. Sure it was glitchy, and it was unbalanced, and it was plagued with teleporting knifers and quickscopers but you know what? It was still fun. The matches always felt so fast paced and hectic, with tons of air support and craziness abound. Black Ops, though more balanced, just feels boring in comparison. Plus, the currency system completely kills any desire to keep playing the game post-prestige. It would be one thing if it only applied to attachments and mods for weapons and such, but having to buy EVERYTHING is a pain in the ass, especially considering half of it isn't even available until a specific level anyway.
CoD 4 was and will always be great, but going back to it now its just so simple and barebones that its boring to play. Plus, the community has gone completely insane: most left a long time ago, and the rest act like some crazy guy whose been living in a bomb shelter for the last 5 years and doesn't know that new things have been made. WaW's community is down to nearly 1000 people, but at least its still fun.