Billy said it was 3 Peaks and Edale, as there's so much running involved
Wasdale is considered hardest by many (well, the PBS maps at least) as it's got the distance and Scafell Pike right at the end.
Jura isn't as long but 8000' climb in the first 11 miles, then THAT road.
In terms of stats, I believe Great Lakes is the only race (outside of short up'n'down blasts) with an average ascent of >100m/mile.