I'm going to piss off some people by saying this but... BioShock.
Don't get me wrong, there is some aspects of the game that I love. I love the whole setting, the Art Deco style is beautiful, the story is good, if a bit shaky near the end. I pretty much like everything about the game except for one thing.
The gameplay. It's dull. I haven't played a shooter with such boring, unsatisfying combat as BioShock before in my life. There are no satisfying weapons to use (Save for the shotgun, but even then it's not very good) and almost no variety in enemy design. Plasmids really weren't fun to use either. I could almost get over that, as there isn't a huge amount of combat in the game at first, but after the big reveal, the game decides to up the ante by sending endless waves of splicers after you and it gets really, really boring.