Oblivion and Dragon Age
Oblivion was by far the most boring rpg I have ever played. Silent Character, bland main story, bad game design choices like the leveling system... after more than 30 hours of pure boredom it seemed to me that the whole game only consisted of sidequests. There was not one single part of the game that I liked. Since it got so much praise I expected a great game... therefore it was my biggest dissappointment.
Dragon Age... I started the game with the Elves because my wife is a big fan of fantasy stories with elves... after 30 minutes my wife felt asleep because the whole story was told in such a boring way.