Campaign wise? Probably CoD4 or Black Ops 1.
Multiplayer wise? Its a direct tie between Modern Warfare 2 (the REAL MW2) and Black Ops 2, as its fun vs balanced.
Modern Warfare 2 is absolutely batshit insane because every gun is beyond broken-feeling (everything kills so quickly, some guns are laser accurate while others have strong recoil but kill in like 2 shots), but it ends up equalizing and every gun ends up feeling equally viable. The maps are all largely great, the killstreaks feels super rewarding to earn, its just a FUN multiplayer because of how ridiculous everything is. It's a complete fluke everything works out. Also playstyles are all equally viable, you could be a maniac sprinting around dual-wielding shotguns, pistols, and smgs and top-frag, or sit in a corner with claymores and one man army and rule the room you camped in.
Black Ops 2, however, is the best designed multiplayer. Everything is deliberately balanced well, unlike MW2, the maps are easily the best in the series, the scorestreak and pick ten system are both equally amazing, the weapons all serve a purpose and there's no real "bad" weapon, its really just a superbly designed multiplayer arcade shooter. Additionally, all playstyles had counter play, even people who would camp in rooms with shock charges + mines could be hard countered with Engineer + Black hats. Unlike MW2 where matches can snowball hard, Black Ops 2 keeps everything very balanced and rewards team play and objective play with the scorestreaks. The only modern CoD game that even came close to this was probably Cold War.