Oakmont and Olympic are really tough cos of the putting. Conversely BPB has flatter greens so you can get away with bad approaches.
If you include the Trugolf courses the hardest is Whistler for me, again because of the greens, although there seem to be great scores shot there!
Congressional depends on the pins - can be really evil with bad placements. Personally I find Kia and BPB easiest. Have had several all birdie F9s and B9s there.