Shasta & Lassen
While I realize most people in No Cal are based in the Bay Area, I wanted to start a thread on the area of far northern California, specifically the Cascade mountains of Mt. Shasta and Lassen Peak, and the Modoc, Shasta, and Lassen National Forest areas. This is one of my favorite places to visit in the state by bike, and the peaks have tough, challenging climbs to ride. Specifically through Lassen Park from the South (either way, really) and the Everett Memorial Highway on Shasta.
There's also a terrific single lane paved road called Old McCloud Road that goes from the town of Mt. Shasta to the ski area on Shasta, which also drops down the Park Highway to Route 89, which can be ridden for a loop (89 is a highway, but traffic isn't bad).
The ride through Lassen National Park from the 36/89 junction, over the pass, and down to Manzanita Lake is 38 tough miles, one way. I never had the gumption to do an out & back of all 76 miles in one day, but have ridden both ways through the park. Incredible scenery! Leave your car behind if you really want to experience this great national park.
There are also a lot of really quiet roads between the two peaks, plus to the east of there. The Harris Spring (old Volcanic Legacy Scenic Byway) is one, as is Lookout Hackamore Road. There's a road off 139/140 that was once named A1 that my dad showed me once (by car) that was red. I'll be darned if I can find it on a map now, but talk about remote. The Warner Range east of there is also very quiet.
The entire thousands of acres would also be great to visit by gravel bike, or MTB as well.