The toughest shot at St. Andrews is out of the Road Hole bunker after you have putted into it!
I've played the Old, New, Eden, and Jubilee in tournament competition, and for me the hardest shots are:
1. Tee shot, #17 Eden--Road Hole/Principal's Nose hybrid, for me this is the toughest shot at St. Andrews
2. Approach shot, #17 Old--Road Hole
3. Tee shot, #15 Jubilee--pictured above, the tee shot is harder than the approach
4. Par-3 #8 Eden--toughest par-3 shot at St. Andrews; high elevated green set against a sandhill with massive false front and drop-off left and long
5. Par-3 #11 Old--the Eden Hole
Honorable Mention:
Tee shot, #6 New
Par-3 #9 New
Tee shot, #8 Jubilee
Tee shot, #14 Jubilee
Approach shot, #2 Old
Tee shot, #4 Old
Tee shot, #13 Old
Tee shot, #3 Eden