I'm going to 'twist' Sky's thread title just a little. The "biggest" sport upset I've seen (biggest meaning most important to me) was the Duke/UNLV semi-final game. UNLV was a juggernaut, having destroyed us a year earlier in the championship game by 30. And by the time we got another chance at them (they had won 45 or 46 straight). They were dominate. We had our hands more than full (even though I believed). UNLV didn't play badly in that loss, it's just that Duke wasn't intimidated, and we hung with them...something they weren't used to. It was an incredible game, and of course led to us finally getting our first championship.
OFC