TarheelsRule:
From my point of view and my tees and green speed, there is no doubt that Oakmont #9 is the hardest hole on WGT by far. If the wind is up on 17 at STA it can be tough but the putts you end up on 9 at Oakmont make it tougher.
+1 ... Oak 9 is always a tough par; above the hole is a 3 putt; lay up and it's still a difficult up and down, with that weird hole location. BPB 15 can be a birdie; with the right approach you leave yourself a more or less straight, steep uphill putt. St.A 17 you can land way short in a headwind and the ball funnels toward the hole. Neither is easy, but Oak 9 is much worse.