OU/Nebraska
i know, i know, i'm an OU homer, but just wait till these teams are both in the top 10. the Big 12 really needs to find a way to let these two teams play every year.
OU/Texas is a good one too, but i would not think of rating it ahead of OU/Nebraska.
despite what some people like to argue, OU/OSU is more a mini rivalry. It's huge for OSU and once every 10 years or so they get to storm the field, but it's not really that big of a deal for OU.
Outside the Big 12, i'd go with OSU/Michigan, even if both teams are in a down year. Something about watching a college football game at that time of year with over a 100,000 fans packed into the stadium.
USC/Notre Dame, but only if both teams are good.
Texas/Texas A&M. Talk about the school of hard knocks when these teams play, even if it's a one sided game.