Knights of the Old Republic II
Yes, it had the awesomest villain since ages, yes the first part of the game had some pretty brilliant moments (like the HK-50 feigning innocence at the Peragus Mining Station) but I will never, ever forgive Lucasarts for forcing it out of the door, simply leaving the game unfinished and mutilating what was looking like a brilliant game.
I've played bad games, and they're usually fairly constant in their suck, which you can get used to and tolerate. But KotOR 2 is just the biggest dissapointment in my long gaming career.