Of the ones I actually saw, probably the Cheerios Bowl.

Underrated as an upset. We had lost to Tennessee at home after blowing a huge lead, gotten blown out by Penn State, and then squeaked by Hawaii by 6 points. The defense was a disaster, and morale was in the toilet. A lot of experts felt like we would get destroyed by Spurrier's Gators.

The upset win planted the seeds for a really strong turnaround through the next 2 seasons.

1998 over Michigan was surprising, but you can't label it "great" given what happened a week later, and the underachieving season in general.