This is probably unfair since it's not so much "difficult to clean" as "impossible", but I'd go with the walk-up at Iron Cross 2015 in Williamsport, PA. I'd bet that there isn't a rider on the planet (including someone like Hans Rey) who could clean the old walk-up at previous Iron Crosses in Michaux, and the one in Williamsport is steeper and longer and ends with a section you have to literally scramble up a rock face. And just walking up was the most challenging part of the race last year. Calling it "a technical climb" is kind of like calling a cliff "a steep climb", so like I said, it's probably unfair to use that one. But when I think of intimidating sections of any trail, that's the first one that comes to mind. I was so happy that I didn't know if was coming last year, and this year my plan is to give myself a memory-erasing head injury before the race so I can't remember it's coming. Because ... STRATEGY!
As far as actual rideable hills, I might say the Gazebo climb at Wissahickon is up there, but only for singlespeed. If you can stay in the saddle, it wouldn't be too bad. And it's very short. But I think I could count on one hand the number of times I've cleaned it on the single and I've attempted it hundreds of times.
Other than that, not sure.