By far, The Force Unleashed. The original game was a self-contained affair that proved to be a multimedia experiment in exploring an untouched portion of the Star Wars mythos. And a successful one at that. I loved the game, got the graphic novel, and all was well. That was it.
Unfortunately, money spoke louder, and an absolutely needless "sequel" was crapped out, one that added nothing to the story, and couldn't have been any more half-arsed than it proved to be.
What annoys me is that in their previous, similar experience with Shadows of the Empire, they called it all a success and moved on. No sequels, no more tie-ins, that was it, story's done, move on. Yet this time, they done fucked up. Pity.