I suppose we all have many difficult holes on all four courses.
For me it's:
Kiawah - 4, 9, 18
Bethpage Black - 7, 10, 15
St. Andrews - 2, 13, 15, 17
Oakmont - Pretty much the whole course is difficult for me except for 2, 6, 11, 17. I've had a bad taste in my mouth with this course since "The Championship". My best Tournament score was a 68.