In my opinion, if you truly were going to rank courses solely based on difficulty, most of the ones at the "top" of that list wouldn't be the greats, with a few exceptions. Pine Valley has to be up there, and while I haven't played it I would think Oakmont does too. A lot of these other ones, like Pinehurst and Spyglass, are hard but nowhere close to "hardest" level. Cypress Point and NGLA shouldn't have even been candidates for "hardest."
I don't remember which courses I nominated exactly, but I know I went to my spreadsheet and picked out all the courses with 155 slopes and listed them first--so I must have included Pine Valley, TPC Sawgrass, Oak Tree National, Promontory Nicklaus, The Concession, Conservatory at Hammock Beach, and Rich Harvest Links. I believe Butler National, Cog Hill, and Wolf Run rounded out my top ten. Dye's courses are also reasonable selections. I think The Honors is as tough as any of them, but Whistling Straits is a reasonable selection as well. Idaho Club was considered but didn't make top ten hardest mainly because the tip slope was ONLY 148. Blackwolf Run has been extended since my visit, but I found it more reasonable from a middle yardage than some of the others.
Hard, IMO, has very little correlation with good. Some of those courses listed are ones I'd love to see anytime, but others I'm not in any hurry to get back to. Haven't played Ocean Course yet, so not sure where it should fall.