I picked Crouch because the OP's framing isn't 'who was a great QB' or 'who was the best QB' but the 'best qb to ever run the option at NU.' I interpreted that as which QB ran the option the best.
I can't think of any Husker QB I'd rather have run the option than Crouch. It's probably Crouch then Frazier. I think Crouch was the better runner overall, but that was also because the '01 team depended on Crouch more than Frazier's teams depended on Frazier. That's not to downplay Frazier's significance, but I would wager the '01 NU team doesn't sniff the national title game without Crouch at the helm.
Interesting as this is the exact reason I don't pick Crouch. Frankly, we've had several that were far better pitching the ball than Crouch, who tended to have to stare down his pitchman. In fact, I'd go so far as to speculate that with another QB (say from this list) we win the title in 1999. That is, in my opinion, one of the best teams we ever had and the only thing that stopped it was all of those fumbles, many of which came on option plays.