Whiteway just outside Dursley gets my vote. As well as being steep it's ridiculously narrow, so you can't really weave to even out the gradient. Because it's in the shady woods, the road surface is nearly always damp and slippery, so maintaining traction with both wheels is tricky. I once met I car when descending it - although I managed to stop in time, dismounting was challenging!
That raises memories of going up there one November afternoon, dead leaves all over apart from the car wheel tracks (about 12" wide each), and after 3 pints of Old Peculier at the New Inn, Waterley Bottom
. I failed when the front wheel just carried straight on at the first sharp bend, despite being aimed about 30 degrees right.
My memory of Wrynose and Hardknott is that whichever you do first is the harder; not surprising given that the middle is 100 m+ higher than either side. Hardknott from the west was harder than Wrynose from the east, so that will get my vote.
I've not done Bwlch-y-groes, Great Dun Fell or The Mighty North Hill, have done the others. The hard side of Bwlch-y-groes is from Dinas Mawddwy, to the SW.
I sometimes got an early start on the Corker (to be the secret control at Guiting Power), so I got to ride Bushcombe Lane on my own, without being balked by walkers, like I usually did when I wasn't the control.
Winnats is OK with proper gears as a regular cyclist, but it sticks in my mind from when I was a potholer rather than a cyclist, and rode my commuting bike back from Sheffield to Manchester Uni that way. 5-speed pseudo-racer (flat pedals, suicide levers and all), on 46x28. I was surprised the handlebars didn't break, and I collapsed in a heap by the top cattle grid for about 20 minutes before I was fit to carry on.