I suspect that the daddy of them all is Bealach-Na-Ba, though I haven't had the honour of riding walking up it.
I don't think so.
Although Simon Warren rated it as 11/10 in his book "100 Greatest Cycling Climbs", the rating system is not just hard-ness, it's a bunch of other factors too which add to the enjoyment of the climb.
I've never cycled it either, but I've been there inna car, and on foot, several times.
And I've talked to many local cyclists who have done it, and who have been able to compare it to local climbs.
It's a tough climb, for sure.
And it kicks up at the end.
But it can't compete with Hardknott for sheer sustained silly ( 30%+ ) gradient.
I'd have to say that Wrynose comes a close second, because it hits similar gradients, just not so sustained. But it also comes shortly after you've done Hardknott, and are 100 miles in on the Fred Whitton route!
It was with some regret that I didn't detour via the 40% climb in Harlech on the BCM600.
But it was my first 600, and I wanted to keep my powder dry.
Also, I don't think I'd brought the right gears to the show.