Cascade River Road, without a doubt. Something like 4,500 feet of gain in 23 miles of rough dirt and gravel, with some 100 F temps and no shade thrown in, early in the season when I wasn't used to the warm yet. Got minor heat stroke, and took a snow nap next to a bear. Took me an hour and a half to ride back
down.
But I'd been wanting to do it for years.
