COD4 for me. But I didnt get into its multiplayer initially. In matter of fact, I was not a big fan at all. I sucked so horribly at it and the concept of having a multiplayer profile that you level up and unlock weapons was a new concept to me. Up to this point everything in FPS games came already unlocked aside from some maps.
With that being said, it was MW2 when I really started getting into COD multiplayer and COD in general. I really started enjoying it a lot. Once I got better at COD through MW2, I went back to MW1 and got up to level 55 on it and got decent. But that was about it. And then I ended up buying W@W before Black Ops and didnt care for it very much outside its campaign and zombies. Then got Black Ops 1 and loved it. Then MW3 and hated it. And finally BO II and I really like it. I also went back and played COD 1-3 somewhere in that time frame as well.