Toughest holes, in my opinion and I'm basing toughness on how hard it is to make birdie on them.....
1) 5th hole at Merion with back pins, can't get within 12 feet
2) 4th hole at Pinehurst sometimes you can't even reach it in regulation
3) 6th hole at BPB (the long par 4 might have hole # wrong), front right pin, hard to stop
4) 9th hole at Congressional front right pin placement
5) 1st hole at Chambers Bay, long shot in and 3 feet past the hole won't stop
I'm sure I forgot a few but those 5 are not easy birdie holes for sure.