That's hard to say.
MW2 for me was the first COD game that wasn't better or at least as good as the previous. It is the first and only COD that can make me rage out of frustration.
The 'best' or most groundbreaking COD game is probably COD4 (MW1).
For me, I'd have to go with COD2 tho. I had the most fun with that game. I played it for years. Unlike MW2, it would never piss me off. The community was awesome as well, and playing on the same servers all the time really made the experience that much more fun. Without a doubt, COD2 was the most balanced. The most skilled players would win. Period. That has not been the case anymore since mw..