Points well taken gentleman. Here is my take on the toughness of Sandwich's 15th:
There are two kinds of tough for a golf hole. One is the brutal, beat-you-over-the-head type of toughness that makes the hole thoroughly unenjoyable to play. These types of holes are overly long, too narrow, and laden with hazards that make the hole unplayable for all but the lowest handicappers. I abhor these types of golf holes. They are no fun to play, and they do not inspire me.
The other is the maddening, subtle type of toughness. This type relies on more subtle hazards like short grass, unusual undulations, and well-placed bunkering. These holes are very difficult, yes, but they remain interesting and eminently playable for low and high handicappers alike. These holes have a certain type of charm and quirk that make them endearing and timeless. The 13th at Prestwick is this type of hole for me. Prestwick's 13th is very difficult--too difficult, some might say, for a par four. The green is outrageous and makes achieving a score of 4 very difficult. Yet the hole is pure fun and very playable for all levels of golfers. The green is unfair by modern standards, but is there a better feeling than hitting a running pitch to within five feet on the treacherous 13th? I think not.
For me, 15th at Sandwich falls squarely into the second category of toughness. The green is extremely difficult to hit and hold, as you say Philip, even for a running approach. However, that was one of the things that made the green so appealing, especially on a links course. Many links green sites make it clear that you can and should run the ball onto the green for the approach. At Sandwich's 15th, it was never clear what the best strategy for attack was--in fact, it still is not clear to me. The green, like Prestwick's 13th, is tough for a hole of its length. It doesn't make sense for a hole of its length, BUT THAT'S WHAT MAKES IT SO FUN! Furthermore, any level of golfer can play Sandwich's 15th. The hole is long, but it is wide open from tee to green. The golfer can play safely to the right on his second and have a decent chance at a four. There is no type of player that cannot play the hole.
Philip, I suppose I would put Foxy in the same category as RSG's 15th without making claims on which is the better hole. I've only played Dornoch once, and I didn't quite know what to make of the 15th my first time around. I'm sure I would grow to like it. However, I think I found it less playable than my above examples due to the penal nature of the tee shot. The approach to the green at Foxy is so unique, so shouldn't everyone get a chance to hit it? Also, I think the greens on all three holes are outrageous, charming, unique, and great in their own ways.