Hm...I'm not sure. I think the biggest would have been Aliens: Colonial Marines. I bought that at launch, even though I saw the warning signs and my friend told me it was getting terrible reviews. Of course, I wasn't going in expecting greatness, so I didn't get completely burned, but still...
I guess Mass Effect 3 was the one that hurt me the most, and as such, I'm very leery of BioWare's Mass Effect team right now. And that stupid video they showed at E3 did nothing to ease my doubts. Which means I'm trying really hard not to get psyched up for Dragon Age III. I see all the same promises, all the same claims, everything that they also promised with Mass Effect 3...but blast it, I'm still getting excited about the game. I didn't think Dragon Age II was all that bad, so to me the series is still worth it, but if it goes the same way as Mass Effect, I fear I may be done with BioWare.