Two Worlds. My close friends and I bought this for the sole purpose of co-op RPG. We were quite excited for this title for that reason alone. The co-op/online component was so terrible though, so laggy and unplayable, that everyone except me returned it immediately. I played through it later, despite the terrible controls for steering your mount and other issues, and a forgettable story, and 100%'d the XBL achievements. I'm somewhat ashamed to this day. I should have just 100%'d Kung Fu Panda.
Edit: Honorable Mention to Medal of Honor: Airborne. The SP was...not what I expected. It was like they took what was great about the MoH series, the MoH WWII realism, and threw it out the window for something closer to what you'd see in iteration of Wolfenstein (I refer to it as Wolfenstein-lite). The MP was simply god awful - 4v4, terrible gameplay performance, terrible net-code performance, and terrible gunplay. Yet I stuck around long enough to get all achievements except one, a 0 point achievement that is glitches and unobtainable for most players - WWII Victory Medal I think it is called.