I am somewhat amazed by all the responses and the course left out.
Surely(and I've played these), Oakmont, Medinah, Butler Nat'l., and Ko'olau, are very tough and difficult without any "element impairment"....i.e. wind, rain, temp, etc.
I'm know the Shinnecock's, Bethpage Blacks, Kiawah OC, and Muirfield can be brutal with any appearance of wind, btw ....as can be Royal St. Georges as well.
But can anyone who's played Carnoustie on any day (with or without the normal sideways rain and wind and with or without the Royal Marine rifle-firing symphony) tell me its not THE TOUGHEST.....Maybe I am crazy, but that course just goes brutal from one-to-eighteen with little, if any, let-up.