I'd pick Grambling vs. Southern too. Gotta love a rivalry where no one gives a crap about the winner of the game, but instead get into big wars over the bands. That's good stuff.
On a national level OU vs. Texas is bigger than Texas vs. A&M, but....
--for people who grew up in Texas, it's one of those where you pretty much pick a side early on in life you're an Aggie or a Longhorn (then the majority of us get to college age and become neither but still have a preference)
--in state, almost everyone in Texas has a mutual disrespect for Oklahoma, the schools, the state, the people (though over the last 10 years or so, we've made the Indian nations rich by flooding the border casinos. Southern Oklahoma is now home to the world's third largest casino).
---outside of football, Texas vs. Oklahoma is useless, except maybe in softball. Every other sport it's pretty much Texas vs. Texas.
All that being said, I don't know I'd put any Texas rivalry ahead of Bama/Auburn or Ohio State/Michigan.