Easily Assassin's Creed.
While that game had some good elements in play, it was overwhelmed by bad design choices, horrid combat, badly implemented stealth mechanics and a world, that while interesting, was only there as a climbing surface and nothing.
Even these flaws I could have forgiven if the last part of the game wasn't the worst aspect of the game for three chapters followed by something that if you actually are willing to call it an ending you are bing FAR to kind.
Also, though I have not beaten it, I was also pretty let down by Super Paper Mario. Depending on how oyu look at it it's either a RPG with a boring, easy and simplistic battle system or a platformer with gameplay that crawls along at a snails pace. Also, if you are making a mario game and your dialogue to gameplay ratio apporached, let's say, Xenosaga levels, you should start over. No one that's worth mentioning cares that much about the plot of a mario game.