Dang I'm different from all the posts above me..
IMO Mw2 had the best maps. Could be tough fighting anywhere.. Mw3 and Blackops2 maps starting moving away from having players be able to go in corners, putting obstacles there and such. And jumping up onto objects that a real human being would be able to get on top off. It takes away from the realism, although it does take away campers. But i mean if you're good you'll kill campers either way. I hated the messed up sniping in Black Ops 1.. really glad BO2 brought quick scoping back. I'm quite content with Black Ops2 so far though. My list would be ranked like this, with the main attribute being the most fun game to play, and the best memories i have from each
Mw2(without the hacking.. it was just a FUN game to play with friends online)
Mw3.. or Black Ops2. Almost a tie.
Cod4(force host was the biggest issue)
W@W
Black Ops1