James,
That's a tall order, but I'll give it a shot. It will end up being rather subjective I'm sure. As the thread progressed and as I thought about it more---rather than quickly jotting down the ones that came quickest to mind, I'd add some holes. Jim was exactly right to add C-2 at Huntingdon Valley and the 13th at Pine Valley is certainly worth considering.
Long Par 3: 3rd at Merion (new back tee)
honorable mention: 15th at Philadelphia Country Club
I was tempted to add the 5th at Pine Valley, one of my all time favorite holes (but not even the best 5th hole in our district) but did not because any shot short of the green leaves a relatively easy bogey. Of course, anything left, right or over is serious trouble.
The reason I chose the 3rd at Merion over the 15th at PCC is the green. At Merion, you can see the first portion of the green but in no way does it appear the green is 48 yards deep nor the bunker below the green is 35 yards long. The scale of both makes it difficult to judge by sight. Merion's 3rd green is so hard, there aren't any easy putts. You can over read, under read and completely misread the green. From the back tee, you can hit anything from a 6 iron to a driver depending upon wind and pin position. The shot to the front of the green is 216 and to the rear, 264. Philadelphia Country's 15th is a blind shot, you can only see part of the flag. It is seriously uphill and treacherously bunkered. There is a subtle front to rear spine that makes reading the break very difficult. I think it has been aced less than 10 times in 80 years.
Short Par 3: 11th at Shinnecock Hills
honorable mention: 2nd at Royal Dornoch
The skyline nature of the 11th is being returned and this makes for a very uncomfortable tee shot knowing there is danger looming all around that tiny little green with severe slopes. The 2nd at Royal Dornoch comes closest to being its equal at about 20 yards longer.
Long Par 4: 18th at Merion (new back tee)
honorable mention: 12th at Swinley Forest
I should have had the 5th and 18th at Merion tied for first. They are among the greatest and hardest holes in golf. The unique 18th green (front half back to front, back half front to back) and the demanding 5th green along are the finish of very demanding tee shots and LONG second shots off difficult lies. These holes are supremely demanding to all players.
The 12th at Swinley is a gem of a design with a wonderful kicker that you can use on the left to feed the ball onto the very interesting green. I think it is a beautiful hole and extremely difficult. The tee shot to the dogleg hole is very intimidating and the long approach requires precision on the ground or in the air.
Short Par 4: 5th at Royal St. George's (if 420 is short these days)
honorable mention: 11th at Huntingdon Valley
I chose the 5th at RSG from one round of play. I don't know it that well, but I remember walking away from that hole wanting to play it again right away. It was hard, dramatic but fair. I cannot really say why I think it is so hard, it probably isn't the hardest medium par 4, but it is one of the best.
HVCC's 11th is shorter than the 5th at RSG but the approach is one difficult shot with overhanging branches, a center spine on the shallow green and slopes that can feed the ball into the fronting creek. It is one hard hole that requires hitting a small ideal landing area within great fairway width. You had better use your noggin before playing this hole for the first time, or fiftieth for that matter.
Long Par 5: 12th at Oakmont CC
honorable mention: 3rd at Philadelphia Country Club
The recent write-up on the 12th at Oakmont says why this hole is so hard better than I could. It is both strategic and penal with bunkers that are deep and punishing. The green is very difficult once you finally get there. It is a seriously long hole, even if it is downhill.
Maybe the best bunkering on an inland hole. You have to tack your way around this hole like an expert sailor. The bunkers are perfectly placed. The approach shot demand sets up the prior two shots. The green runs away from you and is expertly bunkered. Any approach shots taken from the right rough have to deal with an elevated shot with a large and deep bunker flanking the right side of the green. Approaches that finish in the bunker or below it will leave very little if any of the flag visible. Putts are much quicker front to back than you think and slower back to front. It is a great test of a par 5.
Short Par 5: 16th at Shinnecock Hills
honorable mention: 4th at Bethpage Black
Beautifully bunkered, about as perfect as the inland bunkering of Phila Country's 3rd (it was once the 18th where Snead met his demise in the 1939 Open). It has terrific views of the hillside and clubhouse. Yet the tee to green demands are great, especially so on a windy day. The bunkering closes up the front so that a precise aerial shot needs to be played. Ask Tom Paul about putts from back to front. From the middle of the back of the green, you could putt along the left and right edges and down the middle and they'll end up right near each other. It is hard to believe and impossible to read that way.
While I think the bunkering on this hole and others at BB are over the top in size and shape, this is one difficult hole even with a so-so green. I can think of other holes more difficult, but I didn't want to hurt Phil's feelings
nor did I want to load up the list with Flynn's. I think it is an excellent hole, but I could easily substitute other holes for this.