For me personally, Mass Effect 3. I never played a Mass Effect game up until 3, and considering all the hype for it, I was expecting a whole lot more than I got out of it. To this day, I still don't understand how anyone gets so excited over what essentially amounts to a choice driven segmented cover based third person shooter (especially considering that all those choices made during the game really didn't mean crap during the ending), but hey, millions of people buy Call of Duty each year and I don't understand that either.
For a fanbase as a whole, Metroid: Other M. I still don't think Other M is a "bad" game, and if it wasn't part of the Metroid franchise I might even call it good. However, the hype for that game was sky high, and everything that the game was supposed to do, such as explaining Samus' complex relationship with Adam and delving into Samus more as a character, was botched horribly. Ironically, the one part that didn't get a whole lot of focus, which was the combat, ended up being the game's saving grace (or, at least, saving it from complete and utter disaster).
Dishonorable Mentions would be Dead Island and Sonic '06.
