I didn't really like CoD 2 all that much. I liked CoD 3 because of all the different nations it had. CoD 4 was a masterpiece, and because of that they decided to never change the forumula...
CoD 5 had the best single player, MW2 had a good singleplayer and decent MP, BOI was shite, BOII was my favorite post CoD5 game.
MW3 and Ghosts were crimes against humanity.