Personally, Call of Duty: Modern Warfare remains one of the best shooters ever made. Unfortunately, every year they look at the previous game and say "Ok, how can we make these even dumber so more fucktards buy this thing?"
World at War was a joke of a game, and Treyarch were a laughing stock after it came out. Infinity Ward smashed the WWII FPS Hell to pieces by reinventing their franchise as a modern military shooter. Treyarch were left spinning.
Modern Warfare 2 remains one of the worst games of all time. It's plot was insulting stupid, it's multiplayer so broken and utterly unplayable as to call into question the mental capacity of it's developers.
Black Ops was were the games actually didn't matter any more. Activision Blizzard was the proud owner of a series that people would buy because it was popular. I never played Black Ops. I don't want to, either.
Modern Warfare 3 is, by all accounts, the same as everything before it. By all accounts it's multiplayer is less broken, but it's single player campaign is certainly dumber and less fun. And adding a Horde mode to a military FPS is insulting to everyone involved.