I think WaW, or Black Ops are the best two. I would say that WaW is better if not for all the modders on multiplayer. Modern Warfare two is pretty cool too MW3 would be awesome if not for the lag issues. They are all pretty good but there are drawbacks to each title you just need to decide what quirks you can tolerate in the game. Black Ops has bad hit detection, WaW is hacked to ****, MW3 has some bad lag issues for some people, and I can't really think of anything with MW2 except I don't care for a lot of the maps although Favela is my favorite CoD map of all time.