Of the courses I've played, I would certainly say that Carnoustie and Muirfield are among the most difficult. I think Merion East's 18th hole belongs as well. It has been lengthened and plays to 490 yards from the championship tee. You have to carry the ball 235 yards to clear the quarry and 245 yards to reach the fairway. From there it is a slightly narrow roller coaster fairway to a green with a significant false front with a continuing slope that short balls invariably roll down and balls with too much spin (usually a third shot) can travel as well. There are large flanking bunkers at a broad opening to a green that is domed along the line of play. The front half of the green slopes back to front and the rear half slopes front to back. Over the green is thick rough and a very difficult pitch back to the green. You are very likely to be chipping back once more from off the front of the green.
While Kittansett has only held a Walker Cup, its 18th is spectacular as is the 18th at Lancaster. For some reason, Lancaster CC has never held a significant tournament. It should!