Halo 2 - Short single player campaign and too much hype, not to mention the guys at Bungie spent WAY too much time on the multiplayer aspect of it. If they were so focused on the multiplayer and online aspect of the game, they should have made it strictly for online, not have it mostly multiplayer with a half-a**ed storyline that was short and unfufilling, in my eyes.
Half-Life 2 for the PC - Steam killed it for me. 'Nuff said.
I'll add more when they come to me.