i guess a couple things come to mind here
1) if we are just talking about a great performance in coming back from a deficit it is hard to beat Minny and LSU by Greg Frey
both had big deficits and lasted till late in the game to come back
2) however my criteria would be a little different
first the game had to end up being meaningful or was very meaningful at the time, so sorry Minny, Iowa, LSU dont count as they were simply come from behind wins in otherwise meaningless game
second we had to trail for a large portion of the game not just have happened to find our selves behind late, especially if we had the lead earlier, so to me that eliminates the National Championship game as we led that game late on the 4th quarter and let that lead slip away for force OT, great win but not a great "comeback"
lastly you had be down by more than 3 points at some point, coming back from a 3 pt lead is not a great come back, it may be a great ending but not a great comeback
so to my mind this severely limits that list
Purdue in 02 and in 12 would both qualify, we were down most of the game, by more the 3 late and came back to win, and both games maintained undefeated seasons
Michigan in 05 would count, although we had the lead early, we fell behind midway thru the 3rd qtr and by mid 4th were down by 9 and of course the game is always big.
the 79 win over michigan is close as we trailed for large portions of the game and won on a blocked punt in the 4th qtr, but the final deficit overcome was only 3, but it did secure an undefeated season, was our first win in 4 years over them and led us to a Rose Bowl and potential national title shot
but as i said if you eliminate the need for the game to be of some importance, then Minny, LSU, Iowa, etc all come back into play