While there are endless climbs around the SF Bay, one thing it lacks are true monsters. Up in Lake County, starting at Lake Pillsbury, there is, as far as I can tell, the biggest continuous cycling climb in the greater Bay Area. 5,400ft of climbing, with a total elevation change of 5,002 feet. The average grade is 8.6% with some sections steeper than 15%, and the terrain ranges from washboard dirt to loose rock/shale. Aside from the main road on the way up, there are miles and miles of singletrack, double track, ATV, and 4x4 trails lacing the mountain. I rode it for the first time a few weeks ago, it's truly relentless, in seemingly regular intervals the road goes from dirt to rock and kicks up above 10% grade. Right as the treeline fades away I hit the north face of the mountain and there was still a lot of snow, so I mostly hiked the last 600ft of climbing, which is a 19% grade for .6 miles. The view from the top was more than rewarding enough though, along with the knowledge you've done what few other cyclists have done.
If you want to give it a go it's about a 3 hour drive from San Francisco, the last 40 minutes being on a dirt road which can be rough at times. During the winter the top is snowcapped and during the summer there are a lot of dirtbikes and 4x4s flying around the mountain, so your best bet is late spring or early fall, or a summer weekday if you don't mind the heat.