It's a great question. The last remaining factor that defines difficulty for elite players is wind. Without it they can reduce 7800-yard courses to pitch-&-putts. Wind was very much in play at Southern Hills for all four rounds. It's the reason the Old Course can be plundered with -20 in windless conditions and level par is great at Carnoustie when it blows. Perry Maxwell designed most of his courses in the midwest, where wind is a constant factor (I'm a member at Davenport CC so I know). The crucible in which to test Maxwell's work outside the prairie would obviously be Old Town Club. I haven't played it (although I'd love to). How does a Maxwell design stand up in a warm-weather, relatively windless environment?
Which other Maxwell courses are designed to test the best players?
Ciao
Prairie Dunes is in the discussion for hardest course I've ever played yard-for-yard. Steve Salmen and I are both former Wolf Run members and we both agreed on that point after a round at the Mashie a couple years ago.
I actually first considered the superlative I tentatively assign to Perry in the OP while at Old Town Club a month ago. It's not an excessively difficult course necessarily, but it can be extremely stern in punishing nearly-good shots. Lots of places where a ball can get within a few feet of the middle of the fairway or pin, but wind up 20-60 yards away by the time it's done rolling.
Augusta and Southern Hills require no further comment. And Crystal Downs may be shortish for the elite player by modern standards, but yard-for-yard, it's not a slouch at all.
That covers the courses in his catalog I'm most familiar with, but I don't see many limp-wristed handshakes in Perry's portfolio. Tillinghast might be another real contender here, to Bill's point.