Certainly a big upset, and probably in the tops ten, but here are a few it is not bigger than:
US v CCCP 1980
Sox -v- Yanks '04
Jets Colts
Pats -v- Rams '01 Remember the greatest show on turf? The Pats had no right winning that game but they did. This game, the Giants always had a shot and those that thought it would be a blowout of epic proportions were blowing smoke up their arses. I forgot who I was talking to from here a bit about this (maybe Afterhours??) but I've been saying tight and low for a while but that was tighter and lower that I even thought. "Course I'd picked the Pats to win.