And I bet that answer would be Auburn vs Alabama if they were honest. In fact Army vs Navy would be second. Michigan vs Ohio State may be third ;)
You will also get a unnanimous answer from Auburn vs. Alabama fans. My college roomate was a Michigan native, who moved down for school. He swore that their was nothing bigger than big ten football until he lived here for a few months. He knew better then, it was hard for him to swallow but he finally realized just how epic it is down here. Tenneessee vs Alabama is a rivalry but no Bama fan would ever tell you its as big as the Iron Bowl. The Iron Bowl is the biggest rivalry, in the best conference. ;) But it is obvious both are great battles no matter which one you think is the best!