I haven't played #18 at Cypress, but Paul might have given us the most compelling answer. Among the 8 courses on Tom's list that I've played, there's a clear winner, and it's not even close.
#11 at Merion!
This may be a controversial claim, but hear me out. For me, the hole requires something like a 4 iron off the tee and then perhaps a gap wedge to the green. The smallish green is effectively an island--if you miss in any direction or if don't have enough spin on your approach, you're completely screwed. The bunker is probably the best place to miss, and even then, you could make a big number. But you are hitting a wedge into the green, so maybe that's not so bad. Except the rough is so thick that if you miss the fairway, you have no hope and are probably better off chipping out. That's an odd feeling--being say 115 yards away from the hole with no obscured view but nonetheless chipping sideways, haha. Okay, you say, but you're hitting a 4 iron off the tee, so you should be able to hit the fairway. But the fairway is blind from the tee and tiny, and if you haven't played the course in a year or more, there's a good chance they've moved the fairway since you saw it last!
Even if this hole weren't so silly, contrived, and devoid of strategy, it doesn't really fit with the rest of the course aesthetically. The rock wall and the water works just don't really fit with the rest of the course. If we were at Butler or Medinah, sure, but Merion?
And I can't believe that 5 other holes from Merion have been mentioned on this thread but not 11. 2, 4, 7, 12, and 18 are all really good holes (although I see the aesthetic point about the chain link fence on 7), and one might argue that 4, 12, and 18 are great holes. Overall, Merion is a fantastic course with such a unique look and feel, which is one of the reasons that 11 is so disappointing.
The real reason, I suspect, that nobody has mentioned 11 is its historical significance. Sure, Bobby Jones clinched the grand slam on that hole. That's pretty cool. So put up the plaque and call it what it is--the worst hole on an otherwise magnificent course! I wish I could have seen the pre-1930 version of the hole when they teed off from across the road and the green was short of the creek. Maybe that was a neat hole. At the very least, it probably wasn't so contrived and it probably fit in better with the rest of the course.
Of course, I'm probably exaggerating for the sake of a fun argument. We are talking about the best courses in the world, and the 10s are, by definition, not supposed to have a weak hole. "Nearly perfect; if you skipped even one hole, you would miss something worth seeing." So who wants to take me on and argue that 11 is not, in fact, a weak hole?