About a decade ago, I think it was SI did a survey to find out who was each school's biggest rival. Within the MWC, almost every (if not all. I can't remember exactly) school said BYU was its biggest rival. BYU, of course, said Utah was it's rival and I don't remember any other schools coming close.

I'd be curious to see that survey repeated since the conference makeup has changed so drastically since then.