I believe 9 is the hardest. Can you play fast double notes easily? I heard that it's harder than Feux Follets.Do no.6! im doing it right no now and it's beautiful and not too hard. Watch out for the left hand though.I recommend no.7-if you can pull it off. A truly amazing piece.
Feux-follet and Prelude op.23 no.9 are indeed comparable in difficulty.
You must be joking, Thierry. Are you?
Try both.
How do you know I did not?