Halo. Call of Duty 4. Diablo 2.
Reasonings.
Halo: I enjoyed the first part, with the fun enemies, and the pretty landscapes and everything, but once they introduced the Flood it suddenly got shitty. All of the sudden the enemies were boring, there were stupid design decisions, the levels got insanely constrained, even when you were outside, and they were bland. Fist half was fun, not great, though.
Call of Duty 4: The only Call of Duty I've played, and it is a good game, a genuinely fun and good game. But it does not deserve the amount of praise it gets. It's not that amazing, guys. Infinity Ward aren't that great a writing an amazing story, merely decent.
Diablo 2: My main beef with this game? It lacks the atmosphere of the first one, and the atmosphere was what made it great. The gameplay was fun in the first one, and 2 does improve on that, but at the expense of the sense of dread that pervaded the first game.