For me, the Halo series. Part of that is not owning a XBOX, but when I played it on other people's consoles, I never really got into it. I never tried the single player, and the multiplayer seemed lackluster, especially when compared to shooters such as Red Faction (the first one, not the second one) or Call of Duty. I wouldn't say I hate it, I just don't see why it gets so much praise. Metal Gear Solid is another, and frankly after Yahtzee's review of MGS4, I'm not about to give it a whirl. Possibly GTAIV as well, but mostly because of the stupid socialization element more than anything else. I much prefer 3 and Vice City. Finally, Starcraft. Part of that is invariably due to the fact that I suck at it, but that notwithstanding, I still prefer C&C, Homeworld, SupCom, and well, really any other RTS to this one.