Gotta be the E100.
Almost 95% singletrack, arduous climbs, fast descents, predawn start, everything you're asking for.
I'd say even though the Lumberjack had a high dropout rate it's for other reasons than difficulty. Convenient quitting. You passed by YOUR CAR twice every lap. Yes it was hot and miserable, and the course required constant pedaling. For me it was kinda boring. I like to climb and descend, and there was none of that.
Why did I quit after 50 miles? I spent the previous night in the back of my Element with my drunk and snoring friend. It was a kajillion degrees that night, and I couldn't even crack a window because Michigan's atmosphere is 5% mosquito. I didn't sleep. At any other race quitting would have been hard, because I would have had no idea where I was or how to get back. I considered laying down in the woods for a nap, but the ground was teaming with ticks so it was not an option.
Now I'm not saying they didn't promote a good race. Just definitely not my cup 'o tea when it comes to hundies. I always judge the difficulty of a race by how many times I wished I were dead while out on the course. While the Lumberjack did suck away at my will to live, I didn't ever wish for a quick death. I just wished I were doing something else.
At the E100 I wanted to die numerous times. There were feelings of desperation, isolation, being lost and far away from the next checkpoint and civilization. That's how I want to spend my money.