Okay, let's face the facts here.
Who plays Call of Duty for the singleplayer? The only ones worth playing alone were the first and second one, both in WW2.
Call of Duty 4: Modern Warfare had an excellent story, that was presented well, but it doesn't matter because the multiplayer was great.
World at War was just all around a terrible game.
Modern Warfare 2 had an unbelievably stupid story, that was still presented well, but again, it doesn't matter because the multiplayer was okay (not as good as 4, see "no dedicated servers", "overpowered perks/weapons").
Black Ops had an unbelievably stupid story, that was presented terribly, but it doesn't matter because the multiplayer was great (goes back to what made 4 good, and removes stopping power/martyrdom/juggernaut-type perks).
So, Black Ops has the best multiplayer, Call of Duty 4 was one of the best games of its time, and the rest of them (except 1 and 2) either suck, or simply aren't as good.