Not the absolute steepest, but a popular NorCal killer is Bohlmann-On Orbit. It's a 4.5mileish ride that 'only' averages about 10%, but since a good chunk of the early part is mellow, it's way, way worse in the latter half.

Once it gets going, you're hammered with 13-16% for a good distance and then rewarded with a near-continuous 20-21% stretch at the very top. Nice.