Really, REALLY old-timers might say Dick Felt or Virgil Carter.

Really old-timers (I'm in this group) remember fondly Gary Sheide QBing BYU to their first-ever WAC title by unseating ASU in 1974, followed by Giff Nielsen racing away with the 1977 Heisman race until the Oregon State game.....BUT most from this era probably are right with you about Jimmy Mac and Steve.

Still younger fans who didn't see McMahon/Young might revere Robbie for QBing the National Championship team.

Then would come those who saw Ty play in person...and the youngest, most recent of Cougar fans might make an argument for Taysom.

I really think people's answer to your OP would depend in large part on which Cougars they actually saw in person as opposed to in old media clips and archived broadcasts.