I agree with YDRMV. Here is my ranking for the difficulty of Northern California double centuries:
1. Terrible Two
2. Devil Mountain Double
3. Alta Alpina
4. Knoxville
5. Mt Tam
6. Davis
This does not correlate with my finishing times, but more with how hard it felt at my lowest point in the ride. Last year's Knoxville staff ride we had rain for the first 7 hours. The year before the temperature was in the high 90's in Loch Lomond climb, right after lunch. Similarly, Alta Alpina has 5,000ft more climbing than Terrible Two, which compensates with steep climbs, potholes and brutal heat. Only 35% of the field finished on time two years ago.