I'm cheating by picking 2 done one after the other on the Dragon ride 2019 , the Blwch (not Blwch y gross ) 5.8km at 6% steepest 22% descend that then climb Rhigos 6.2km at 4% steepest 7% then a super fast descent best 15 miles of riding I've ever done
The Blwch comes quite near the start so riders are still fairly bunched up halfway up there's a hairpin bend where you see the top and bottom and the road was covered by hundreds of riders in the sunshine which was quite a sight
Presumably that was going up the Bwlch from the Port Talbot direction. I've been down that way, but only gone up (twice) from Nant-y-Moel. I do like that climb. Quite long, but not steep, you can just settle into a rhythm and keep going.
Another, very different, climb I like is Tyn-y-Coed road up to Pentyrch from Creigiau. Only a Cat 4, rather than the cat 3 of the Bwlch, but because it is a bit shorter, and variable in steepness, probably harder.