Oh, you sweet summer child...
First of all, welcome to forums!

Second of all, LOTS of pieces are harder than the S. 141 version of La Campanella. Just a few off the top of my head... (individual pieces that are by themselves more difficult than S. 141 no. 3 La Campanella listed in parenthesis, bolded works are notably more difficult than other pieces I've listed)
LISZTS. 140 Etudes d'Execution Transcendant d'Apres Paganini (No. 3, 4,
4b, 6)
S. 139 Transcendental Etudes (No. 2, 4, 5, 8, 10, 12)
S. 137 Douze Grande Etudes (No.
2, 4,
5,
8,
10, 12)
Rondeau Fantastique "El Contrabandista"
S. 253 Spanish FantasyS. 253 Spanish Rhapsody
S. 695 Spanish Songbook
S. 219 Grand Galop Chromatique
S. 218 Galop in A Minor
3 Concert Etudes, S. 144
2 Concert Etudes, S. 145
Concertos (No. 1, 2, 3)
Grand Sonata in B Minor
Annees de Pelerinage (Books 1 and 2, Dante Sonata)
Mephisto Waltz no. 1
Hungarian Rhapsodies (No. 10)
Ballades (both of them)
Reminisces (all of them)
Scherzo und Marche
Symphonic Poems (No. 6 and probably more, but no. 6 is the only one I'm familiar with)
S. 464 Transcriptions of Beethoven's Symphonies (all of them, especially no. 9)
La Clochette
S. 700i
S. 700iiCHOPINBallades (No. 4)
Etudes (Op. 10, Op. 25)
Sonatas (all of them)
Polonaises (Op. 22 Grande Polonaise Brillante)
BEETHOVENSonata no. 29 "Hammerklavier"
RAVELGaspard de la Nuit (both the Ondine and Scarbo movements are much harder than La Campanella, and Le Gibet is arguably on the same level)
La Valse
Miroirs
BALAKIREVIslamey
RACHMANINOFF2nd Concerto
3rd Concerto
Etudes (Op. 39)
STRAVINSKYTrois Mouvements de Petrouchka (the only movement La Campanella even comes CLOSE to is mvt. 2; the others are among the hardest works in the repertoire)
SCHUMANNOp. 7 Toccata
Op. 13 Symphonic Etudes
Op. 17 Fantasy
DEBUSSYPreludes, Book 2 (Feux d'Artifice)
Images (Book 2)
Etudes (No. 12)
BRAHMSVariations on a Theme of PaganiniPROKOFIEVOp. 11 Toccata
Sonatas
Concertos (I believe all of them)
ALKANOp. 76 Etudes (all of them)
Le Chemin de Fer
Op. 35 Etudes
Op. 39 Etudes (I believe all of them, particular attention to no. 1, 4-7, 8-10, and 12)
Symphony for solo piano (Op. 39 no. 4-7)
Concerto for Solo Piano (Op. 39 no. 8-10)
Le PreuxScherzo Focoso
GODOWSKYPassacaglia
Studies on Chopin's Etudes (no. 47, 50)
FAUREOp. 19 Ballade
MEREAUXOp. 63 Etudes (I believe all of them, particularly no. 20,
24, 42,
45, and 60)
SCRIABINSonatas (No.
5, 6,
7,
8, 9, 10)
Etudes (Op. 42)
MOZARTSonatas
SCARLATTISonatas
ALBENIZIberia Suite (all movements)
IVESSonatas (all of them, particularly
no. 2)
CZERNYSonatas (Op. 268)
SORABJISonatas (all of them, particularly
no. 5)
Sequentia CyclicaSymphonic VariationsEtudes (I believe all of them)
FINNISSYEnglish Country-Tunes
XENAKISEvryaliSynaphaiErikthon Sonata
MESSIAENVingt Regards
Catalogue d'Oiseux
LIGETIEtudes (I believe all of them)
And that's just the stuff that isn't disputed!
As for the "top ten," who the heck knows. It's all subjective.
However, there was a discussion about each individual composer's hardest piece(s) about a month back. You'll find some interesting stuff on there.
Lastly, thank you so much for using "piece" instead of "song." Seriously.
Edit: I did not realize how many pieces I'd listed, lol.
Edit2: I forgot to comment this originally, so...
Instead of just focusing on what the "most difficult pieces" are, try to learn them so you can judge their difficulty for yourself. I recommend getting a teacher and asking for recommendations. You can also find some great tutorials for more advanced pieces on this site and on Youtube, for when you get there. Good luck!
