I found this book, which has a neat list of "wild" roads in Washington. Though it is intended for driving, I'm going to try to ride them all, or at least most of them (didn't happen). I guess I might add to it other roads that are especially nice that didn't make his list. I am somewhat surprised that there isn't more organized information about riding Washington's gravel/dirt roads and hopefully this will be useful to someone down the line.
Hurricane Ridge is a big climb but not steep and is easy to pace.
Obstruction point is much harder. It is about 7.5 miles long, with 1,200 ft gained on the way out. The first section is mostly downhill but is very washboarded. That is followed by two relatively short but steep and loose climbs, and one longer one (.7 miles at 7%, 1.7 miles at 10%, and 1 mile at 7%). I was pretty tired already and over-geared with a 34-32 minimum. Going back down was a bit sketchy and I almost lost it once. Doing this with road tires/gearing would be basically impossible, which I guess is why the main Hurricane Ridge Road segment has ~1,700 people and Obstruction Point road has 17.
Going back down Hurricane Ridge Road was nice, although it wasn't steep enough to simply get into an aero tuck and coast, so I ended up pedaling quite a bit on the way down. Still, cruising downhill for that long was fun; especially the twisting bits near the top. The total elevation was 7,800 ft in 52.7 miles.
It is paved for a while and then ascends fairly steeply to a pretty ridiculous vista of the strait of Juan de Fuca.
In Mazama there is an old Cold War era radar station atop Slate Peak. The gradient is never crazy (until you pass the gate up to the very top) and the views are pretty insane. A reasonably rough road to be sure, with some serious drop offs in some places (crashed there ripping down once). I think this is the highest road in Washington. OMTM also has a blurb.