Hardest in terms of what?
Good question and I deliberately left it up to your judgement - the steep, short effort where you're hanging over the bars and struggling to keep cranks turning and the bike upright? Or the long, slow painful death of the Tour de France legendary climbs? I guess as this is the UK the latter type are the ones for this thread, and the ones that I fear the most.
Referring back to the book, there are just 6 that get 10/10
- Dunkery Beacon
- Rosedale Chimney
- The Lecht
- Hardknott Pass
- Wrynose Pass
And one 11/10
Bealach-na-Ba at almost 9km long and with 623m of climbing.
The fact that 2 of the 7 are Scottish suggests that the balance of the book, with just seven total north of the border, is somewhat skewed.
So which one would I say is the toughest that I've been up? Well, I've only faced Hardknott and Wrynose during the Fred Whitton and they were truly formidable being both steep (Hardknott is 33% at points) and long enough that momentum and strength alone will not get you up and over. Even if I'd have been fresh, they would have been seriously daunting.
Would like to see/have a go at Bwlch-y-Groes and Bealach-na-Ba in particular - I've heard real horror stories!