Of course, on some courses it depends on hole placement. The three courses with what I consider the three hardest holes (OAK#9, BPB#15, STA#17) have different pin placements, and some are much harder than others.
Prior to getting to the top clubs, I would have said BPB#15 every time. That hole killed many a good round. I was able to occasionally par the other two, but never BPB#15. I once 3 putted from 5' on the front hole location. But, once acquiring the higher loft, longer top-end clubs, BPB#15 became much less difficult. And the same really applies for STA#17, though it becomes much, much harder once you move to the Legend tees. So, now I have to agree with what seems to be the majority. Oakmont #9 is the hardest hole here.
Funny, there aren't any holes on Congress or RSG that bother me that much. Congress #4 can be rough, especially from the tips, and that par 4 on the back 9 of RSG, I forget the number, with the triangle green, but none of their holes meet the standard of the above 3. Nor does Kiawah #4, which I consider the hardest hole on Kiawah.
Oakmont #9 stands alone.