#1875 -- The Bloody Baron
You use L-1000 and Dole Valley Rd to carve a winding path up towards Amboy, in the foothills of Larch Mountain (no, not THAT Larch Mountain, nor the other Larch which you see signs for off OR-6). This route tops out at just under 2,000 feet of elevation, and my favorite, though usually inaccurate, route mapping site guesses there's over 6,000 feet of climbing in the middle fifty miles of this route.
There really isn't much civilization between Camas and Yacolt (the small town just south of the turnaround in Amboy), and with some rather rough gravel roads you should plan accordingly with clothing, food, and tools. I think the only quasi-commercial building you'll pass over a 20-ish mile stretch is a jail, and that's not exactly where you want to pull up to beg to use their phone if you have a mechanical failure.
There's about 15 miles of gravel in each direction -- heading north, they're mostly (but not entirely) downhill (with some rather steep grades in spots); on the return trip, you get, well, the opposite. Different riders will find one or the other to be preferable. I'd recommend riding it with at least 30mm tires, and will be using 40+mm myself, though if you're comfortable and familiar with gravel on a road bike it's doable on anything.
So -- if you've grown tired of Dixie Mountain and Otto Miller, head up over the I-205 bridge and check this one out.