While Tuna gave us a direction, and Sparano gave us a tough nosed attitude and a very good coaching staff, I think the answer is Pennington. As previously stated, he really changed the demeanor of the team. Also, for all his physical weaknesses, he has not really hurt our team as some of our past veteran QBs (Green,Daunte, Joey, AJ, Gus Gus) had.
and if we didn't have Pennington, Chad Henne would be baptized by fire as he was clearly the best QB we had at that point.