For me, it's GTAIV and Bioshock. Started new saves on them a few times but just dislike them both. It's a shame because I've loved all the previous GTA games, and I really like the world Bioshock made. But that wasn't enough really.
I thought the Fable games had more going for them than they ended up having. I quite liked the first one... Didn't care for the second at all but didn't nessessarily want to like it, even. Didn't play the 3rd and won't play any others at this point.