Ruggers 2007 is the obvious choice. The blowout by Oregon at the Sun Bowl maybe ripped me the most as I watched it, as a 3rd string QB just gashed us.
One that stung a lot was the loss to UCF the year they beat Baylor. Our team was garbage, but we STILL almost beat them to derail their season.
Others I hated losing a TON: UM 6-3 in Ray Jay; the White Out at WVU (Pat's last game); NC State shut out 14-0 in the Meineke Bowl, our first shut out. Of course, losing to UCF last year was painful, but we were 10+ pt dogs on the road, and we witnessed the best indy performance by a USF player ever, as Flowers threw > 500 & fan for 100. It was a great way to end his career in Florida, even if we lost.
Those hurt, but the many awesome wins (WVU a few times, Aub, Clem, ND, USCe, TT, FSU, UM) outweigh the losses. Heck, we survived Holtz...may be never dip like that again.