I believe Griese was 17-5 as a starter, which is good for a 77.3% winning percentage. Rick Leach was 38-8-2, which would be 81.3% if the two ties are indeed 1/2 of a win each. Jim Harbaugh slightly edges Leach with 81.7% (working from old box scores here).
"It's a lot easier being a drug dealer than an AAU coach" - this guy. Tell me something I don't know. I mean, don't think but have never tried either.