In terms of most disappointing? Paper Mario Sticker Star. I don't think it's a bad game and appreciate the depth to it, but it's one of the games I've looked forward to the most in a long time. Everyone was looking forward to the series getting back to its roots after SPM (Also not a bad game) but this game doesn't even have leveling up? Come on. I appreciate the new idea, but this was not the place to do it. The New Super Mario Bros. series is much more in need of innovation.
I sold it already, but Tekken 3D Prime Edition. Had a cheesy movie that made no sense and I don't think it was even cannon to the main story, had a laggy online, no customization except for colors, and virtually no single player content. SSF4 was better and I hate Street Fighter.