Texas in 2005 was a painful loss. The loss to USC in 2009 was painful. The loss to Wisconsin in 2003 was painful (damn you Lee Evans)...I think the Penn State loss during Pryor's freshman season was pretty hard to take as well...I'll take the Texas loss in 2005 as being the most painful.
Yeah, the Florida game is the big one post-season, but can I throw out that for me, the loss to Texas in the Fiesta Bowl was more painful? That was the worst I ever felt.