This was actually a hard decision for me.
Call of Duty 2 still has *imho* the best single player campaign out of all the cods. It was actually the best launch title for the 360 and got the most attention from me till Oblivion and Ghost Recon came out (many months later) but the multiplayer was mainly broken by a rather large amount of lag issues.
Black Ops seems the best well rounded: Awesome SP campaign, and most balanced/mp features to date for a COD game, however...
... Modern Warfare 1/COD4 was the first game to really garner everyones attention and the first FPS on consoles to dethrone Halo in both name and sales. The multiplayer experience, while a bit fustrating and taken advantage by exploiters and modders, was the best thing out there since bread and butter. The single player campaign was a bit short but it was sweet. It also features my favorite COD character(s): Soap McTavish and Captain Price.
It's just that simple.
