I'd say that you have to put the national championship on top since that's what you ultimately play for. After that, it would have to be a rivalry game, conference title or iconic CU sports memory.
1. 10-9 over #1 Notre Dame in the 1991 Orange Bowl for the hardware.
2. 62-36 over then #1 Nebraska in 2001.
3. 27-26 over #4 Michigan in 1994 on the "Miracle at Michigan".
4. 27-21 over #3 Nebraska in 1989 to jump to #1 in the ranks and secure the Big 8 crown, undefeated season and trip to the national championship game (though there was a little business to take care of the next 2 weeks in games the Buffs were heavy favorites).
5. 20-10 over #3 Nebraska in 1986 to break a losing streak to the Nubs going back to 1960 on the immortal "OC Oliver to Lance Carl" option pass.
1. 10-9 over #1 Notre Dame in the 1991 Orange Bowl for the hardware.
2. 62-36 over then #1 Nebraska in 2001.
3. 27-26 over #4 Michigan in 1994 on the "Miracle at Michigan".
4. 27-21 over #3 Nebraska in 1989 to jump to #1 in the ranks and secure the Big 8 crown, undefeated season and trip to the national championship game (though there was a little business to take care of the next 2 weeks in games the Buffs were heavy favorites).
5. 20-10 over #3 Nebraska in 1986 to break a losing streak to the Nubs going back to 1960 on the immortal "OC Oliver to Lance Carl" option pass.