I can't name one. There are several that rank together. The wins over Texas and Notre Dame because of the history. The win over Miami because of the drought since Coach Bryant.
A close second--mostly due to a lot of that "Fear the Thumb" crap--is '08's 36-0 throttling of Auburn.
Honestly, to answer this accurately, I'd have to break it down to wins with Bryant, Perkins, Curry, Stallings, DuBose, Shula, and Saban. There's a game--in some cases several--that are lifetime highlights.
As example: DuBose and the win over Florida--Gainesville and Atlanta. Or Shula and Florida. I'd have to really think about Curry. Perkins had a few. While Bryant would require a novel, Stallings would at least be a short story.