Star Wars: Knights of The Old Republic 2. It made me hate George Lucas for authorizing it. Don't get me wrong, KotOR 1 is one of my alltime favorite games. A very expansive universe, deep strategies, lots of mind-bending minigames, great characters, and it actually uses George Lucas' ideas.
The second one made me lose all faith in the series. Bioware sold the rights of the game to Obsidion (horrible mistake NO. 1) Who initially stripped all RPG elements from the game (horrible mistake NO. 2),took away all minigames (horrible mistake NO. 3) made it extremely glossy (horrible mistake NO. 4 (I love glitchy games. I think it provides the game with more of an edge.)) and at last, the game had the most anti-climactic ending in the history of gaming (horrible mistake NO. 5.) Besides the eerie feel and the great characters, this game sucked