Oblivion.
I could never drag myself through the main missions. It's go here, kill this, talk to him, rinse and repeat. It's wierd as I've completed Fallout 3 and Fallout NV and didn't have the problem, but something about Oblivion really bugged me.
I also completed all the other missions on it, but the main quest - I don't know if it was just something with me, but eurgh it was horrible.
Bioshock 2.
Simply because I could never be bothered. I was just bored and it appeared the whole game was just filler and couldn't interest me in the slightest.
Ilomilo as they never fixed the damn bugfor the DLC so I can't complete every level. VERY annoying. Why make a broken game, then not even make an attempt to fix it? The level freezes everytime I go into it.
And lastly Trials HD. Inferno 2 is the most annoying level ever. That one level makes me hate the game - which is a shame as I really enjoy the earlier, easier levels.