Ally:
How do you define "hard"? Are you interested in things that test the player's abilities, or just the winning score?
Sebonack is the hardest course I have built - especially if it gets windy. We put in long tees I would normally never consider, because the client wanted them.
My third design, Black Forest, was a very hard course, and my perception of it after we finished convinced me not to go that way again unless the client specifically asked for it -- which that client did -- and even them, first to grill them about whether they really understood what they were asking for. There is no course in the world where Tour pros comprise the majority of play.