Some of my favorites ...
#14 Cleveland St over #3 Indiana (1986)
Many credit this game with popularizing the plight of the little guy after the tournie went to 64 teams. I was 9, but a huge hoops fan. One of the most memorable losses I remember as a kid was seeing CSU lose to David Robinson and Navy 71-70 for the right to go to the final 8. I remember racing home from a city league hoops game and sitting about an inch from the TV for this one. CSU also beat a good St Joes team in the middle.
#14 Weber State over #3 North Carolina (1999)
Harold "The Show" Arceneaux. Nuff said.
#13 Princeton over #4 UCLA (1996)
Pete Carill's last game. 43-41 final. The picket fence, and reverse layup by Gabe Lewuelis at the buzzer to win it. Epic.
#13 Valparaiso over #4 Mississippi (1998)
Bryce Drew's three, then face first slide into the floor. With his old man coaching. And in a back and forth classic battle that the whole country was tuned into for the majority of the 2nd half.
#15 Hampton over #2 Iowa State (2001)
Iowa State was all the rage in '01, then went out and got waxed by Hampton 1st rd. That fat slob coach for Hampton going NUTS, kicking his legs while being hoisted by one of his players. IIRC, Hampton also beat Georgetown next rd to get to the 16.
#15 Santa Clara over #2 Arizona (1991)
The world gets to meet Steve Nash. Zona was a top 5 team all year and a fave to win it all. One of just a handful of 15 seeds to win.
#15 Richmond over #2 Syracuse (1989)
Richmond was the 1st 15 seed ever to win, and the Cuse was a great team (Billy Owens) coming off the Keith Smart loss in the Finals.