You're joking, of course. Right? Yes. Of course you're joking.
For me it's Chopin 25/6 or 28/16, I can't play 28/16 too well yet, as you can tell from my post in the audition room. Actually I found Chopin 10/5 as well as Bach's Italian concerto to be just about as hard as either of those pieces.
Edit: wait a minute I forgot, I played ballade 3 last year I think that was harder than those ones.
No I think 25/6 is harder than the third ballade. The 28/16 is about the same as the third ballade.
Hardest songs I've played:
-Beethoven Op.27 No.2 Third Movement
-Beethoven Op.57 First Movement
-Beethoven Op.81a First Movement
-Beethoven Op.106 Third Movement (too long!)
-Beethoven Op.110 Third and Fourth Movement
-Beethoven Op.111 First Movement
-Chopin Op.1
-Chopin Op.10 No.4
-Chopin Op.25 No.3
-Chopin Op.28 No.8
-Chopin Op.28 No.24
-Chopin Op.35 First and Third Movement
-Chopin Op.47
-Chopin Op.48 No.1
-Chopin Op.56 No.3
-Chopin Op.60
-Debussy Clair de Lune
-Debussy The Sunken Cathedral
-Mozart K 310
-Mozart K 457
-Mozart K 475
-Cyril Scott Lotus Land Op.47 No.1
-Schumann Toccata Op.7
I'm really into Chopin, so yeah.