I would say Fleck maybe Cal Stoll with Mason coming behind both.
Cal Stoll. The 1977 season was magical.
Each Gopher coach including Bierman faced headwinds. Bierman had WWII to contend with, and couldn't keep Charles "Bud" Wilkinson on staff.
Skipped George Hauser and Wes Fesler. Others car to chime in?
Warmath kicked open closed doors and provided opportunities for many players (Yes MSU and Notre Dame we were first!). Vietnam War, Desegregation, and decline in interest from the Administration were all head winds he couldn't overcome.
Stoll. Beat both Rosebowl participants including Warren Moon lead Washington in one season.
He was coaching in the Big Two little eight era, but held his own against the little eight and the Pac-Ten. His headwinds were fewer bowls, disinterested Administration, and insanely large rosters of the big guys (Michigan, OSU, USC, Alabama...) in addition to fewer bowls.
Joe Salem actually had a good season when he had the "Throwin' Samoan. He ran a very progressive offense. Mike Shanahan, and Mike Martz amongst other assistant coaches during his era. Top assistant coaches left and his pass heavy O was figured out. Down went Salem.
Gutekunst (Sp?) Solid defenses, and overall decent teams, but sanctions seemed to take their toll. Support for the program started to walk away just like Lou Holtz did.
Jim Wacker. Ran a clean program, but... Still wonder what would have happened had they beat MSU his final season. Would it have been enough (kind of glad we didn't have to find out)? Anyway, if he could have only hired a better group of assistant coaches*.
Glen Mason. Marched us back to respectability. Many great memories. Probably the greatest Running back trio the Gophers ever had. Bad breaks and going the cheap route on DC's (Young David Gibbs was the exception-cheap, but great) pretty much did him in. Texas Tech and the game we don't mention come to mind.
Tim Brewster. Rather than rail on him (very much of it deserved) I will acknowledge that he could sell Minnesota. If not anything else he presented a good case for Minnesota having great potential.
Jeff Horton. Never had a chance to become a Full time head coach, but he gave us a heck of a win vs Iowa!
Jerry Kill. Health, Health, Health, and Mega Tongue.
Tracy Claeys. Scandal, and New AD. The guy was one of the best DC's we've had in some time though.
PJ Fleck. Coyle knocked it out of the park here. The book is still open here, but there is so much hope. Fleck knows how to find assistant coaches* that can get the job done. I know there are question marks around the current DC, but I expect this season to be better. And if not, I think PJ will do what is best for the program.