1. Dragon Age Origins
2. Mass Effect 2
3. Fallou 3
4. Mass Effect
5. Oblivion
Pretty much every single RPG has been dissapointing. Oblivion was no Morrowind, Fallout 3 was no Fallout 1/2, Mass effect had huge bugs and Mass Effect 2 wasn't as open as I wanted it to be. Fable had a shit final boss and FF 13 was so terrible I wanted to rip my own ears out to stop the pain some of the characters in that game caused me.
Dragon Age origins wins because its reletively old-school and doesn't have any huge flaws outside of the fact that it had a limited budget.