Oblivion. I want to go back to it and start fresh. Back in the day I put in something like 400 hours across 2 saves. I liked Fallout 3 but it wasn't even close. Skyrim was fun and I still play it, but I still prefer Oblivion. I think it had some better quest lines.
I have to play more Fallout: New Vegas before I can judge it. I played a few hours of it but got busy with school and never got too far in it.