This is an interesting question, albeit, limited by each person's belt notches. Other than perpetuating the myth that difficulty is quality, I would say...
What makes them hard is getting it close....
9th hole at Wild Horse in Gothenburg. Tee shot is on a one shotter, with bunker front left and hidden bunker behind cut next to center of a green that slopes to the right.
The 9th hole approach at Riviera.
Day in day out, the 13th hole at Bayside (which was aced this weekend by an assistant pro who won the only skin $2880). It was rather exciting because in our group was Scott Holly, who just won the Nebraska PGA, and, his ball rolled right over the cup, stopping a few feet away, almost taking away the skin.
I would guess the road hole bunker explosion shot would qualify, if you are right up next to the wall.? ?