Strange... a lot of people here hating on really awesome games. Oh well, each to their own. My vote goes to Mass Effect. Could not stand that game. The characters were the most dull pointless people I have ever come across in a video game, the graphics either looked totally sterile or ugly, the character designs was boring and unimaginative, the story went nowhere (in the time I played it, which was a fair amount), the dialogue was so overdrawn and tedious, characters moved slowly and controls felt sluggish, combat was awful and frustrating (could never see what was going on and if I wasn't constantly dying then NPC partners were getting in the bloody way), and the interface was incomprehensible. Basically everything that is in a game I hated about Mass Effect.
I have to mention Fable 2 as well. Ugly graphics, annoying characters, HORRIBLE controls, tedious navigation, tactless combat, annoying camera, and the WORST ending to any video game I had ever played. God I hate that game with such a passion. My experience with it wasn't necessarily as dull as with Mass Effect, but it managed to be very boring and incredibly frustrating to play at the same time. Oh yeah, and also the way you have to wait for shops to get the stock in for the potions you want to buy as well. And even once I had waited, a lot of the time the shop owner would just never turn up. Like he was deleted from the game. That drove me crazy, I wasted so much time on that crap.
There's probably other games I found ultra boring, but were so boring I can't remember.