Hard to give one "hardest". Different climbs are hard for different reasons. Three of mine were on hills not that tough. 2 in 1977, my big racing year. The climb out of Skowhegan in the Maine International. Started at the 90 mile point. Very fast race. (We took minutes off a record set by John Howard.) That hill broke the pack. I was the last to make the cut. Absolutely at my limit. Everything I had just to keep that wheel in front of me close. Longest recovery after I have ever seen. And that was just a two mile yet another Maine hill. Also that summer, pushing my riding partner up the two hills beyond Mt Wachusett in Massachusetts. She'd raced the evening before and had no legs. I had to get back to work. It was drop her or push, riding one handed in a 42-19. OMG hard!
The 16 mile climb out of Ashland, OR, day 6 of the 2012 Cycle Oregon. Fix gear, 42-23. Day 4 of 5000+ every day. Two years later, the 2 mile climb day 4 Cycle Oregon that hit 14.5% My photo was taken at the hardest point. Look carefully and you can see my climbing gear (23 teeth) on the left side. So 42-17.) Nobody passed me despite an RPM so low it didn't register. (The bar of soap on my forearm hurt when I showered!)
Mt Washington. Hardest hill, not the hardest I've pushed myself either time. (First time - solo and unassisted. In high mountains, I keep some reserve in case weather changes. 2nd time - the day before was a very windy Cat 1,2 5 hour race in a small field. Not easy for this flyweight. When I hit the bottom of the mountain, I knew a 100 yards in I had nothing, Had to get off and walk twice. But not the final pitch!) And for those who do not know Mt Washington, it is only 6000' but the weather at the top can (and regularly does) become "high mountain" life or death. I'd been to the top several times before. Saw the cross 100 yards from a life saving stash.
Now I am too old to push myself that hard. My ultimate climbs are done or missed. Still climbing but my chainrings keep getting smaller.
Ben