I would say either Mahomes, Allen or Herbert, but I doubt Kellen Moore would have provided the tutelage their current offensive coordinators and coaches have given them. Translation: I don't think even if they were here, they'd be as great here as they are with their teams now. I simply don't see how Moore made this offense better. We were average at best with the talent we had and, possibly, Dak's injury.
However ... the defense DRAMATICALLY improved last year under the leadership of Dan Quinn and the addition of Micah Parsons along with the maturity of Trevon Diggs. Imagine what we could have done with a run-stopper, pass-rusher like Aaron Donald? That's one less scoring drive by the 49ers, meaning a Cowboys win. That's at least one less scoring drive for Tampa Bay, which loss by three points to the Rams. And we're likely playing Green Bay in the NFC Championship Game, who loss to Niners by 3.
Until we get a competent offensive coordinator (or one not learning on the job), our hope is on the defensive side of the ball. Therefore, give me Aaron Donald, and I'll take my chances with the quarterback we have now.