The way I judge hole difficulty is by ability to make birdie and there's no question that Oakmont #9 is far and away the toughest hole in this game to birdie.
Birdie can be had at BPB 15 but I usually on play for par and make it 95% of the time. Simply knock second shot into the bunker front right of green and learn that shot. I've holed more than 10 birdies from that trap whereas I've probably only putted 6 or 7 on that front pin. Have made a couple on the back pin.
On the contrary Oakmont #9 is IMPOSSIBLE to get anything near on second shot. I've also found the best way to play that hole is from the front right bunker and have knocked a couple in from there too. I have only ever made 2 pars when going for and landing on the green. Anything you get on the green is almost a certain 3 putt.
BPB #7 with the front right pin tucked behind the trap is a tough birdie. I've made 4 or 5 but always by draining a 40+ ft putt.
STA #17 is not an easy hole but doesn't compare to those regarding difficulty. Unless I can bomb a drive inside 130yds I play right of the trap and have learned those 22 -30 putts pretty well. I've made way more birdies than bogeys on that one.
I'm sure it makes a difference using master equipment (which I have for so long I can't remember the shots with other clubs) so please factor that into my assessment.