Honestly - I'm not sure who I'd pick right now.
A dominant DL would be awesome for disrupting offenses. Or a dominant WR to make sure the offense moves - one of those guys that is going to get open and is going to take the ball from whoever is defending him.
I think that as much as I'd like to see BYU's offense be super productive, I'd probably pick a DL. Aaron Donald? Maybe a LB like Mack.