It's hard to answer this thread, because if a game is truly that horrible, I probably take it out after 5 minutes of playing and totally forget that I ever played it.

I'll put a spin on this thread and ask, "What's the worst game you've ever finished?" For me, probably Zelda Phantom Hourglass on DS. It was so repetitive and quite annoying, yet I felt compelled to finish it since I was a Zelda fan.

Otherwise, nothing really comes to mind.