For me, it’s got the be the waterbars on Ben Nevis.
By the time I reached the ones around the halfway loch, my arms were completely pumped and I was unable to brake properly or control the front end with any sort of confidence.
Some of the waterbars are absolute monsters. Your concentration levels are already are breaking point after a mile, so it’s sometimes hard to look far enough ahead to deal with them properly.
I took some absolutely monumental hits, getting ragdolled all over the place.
Actually, one of the scariest crashes I’ve ever had was hitting a waterbar on Ben Lomond at about 8pm one summers evening. I was on a motorway section and had good visibility, I was absolutely flat out, must have been close to 35mph, and then I lost the front wheel in a broken waterbar that I was sure I had already cleared (i saw it on the way up and made a mental note to avoid it!).
Sent me absolutely flying and landed really awkwardly on my back. Didn’t break anything (for a change!), but it scared me senseless. Never really had a big stack flat out before.