The Elder Scrolls IV: Oblivion.
The way people talk about it make it seem like a mammoth achievement in gaming history, unsurpassed even by the return of raptor Jesus in Duke Nukem Forever.
Honestly, for me anyway, it started out too slow, the graphics weren't that great, and it just wasn't that fun. On top of all that, for some reason, it froze my 360.
So I guess the only reason I regret selling it is because it was the collector's edition and it looked pretty among such games as GTA IV (collector's edition) and Mass Effect (collector's edition). See a pattern?