For me nothing beats the feeling of playing a Bach fugue. I don’t know what it is but they just feel good to play. They are a pain to learn for me but once they are in my hands I love playing them.
For me, scriabin 4th sonata and any sort of fast beethoven sonata movement. I just find the beethoven so fun to play because of how much character it has. But its not exactly fun when u havent mastered it. i might add Liszt etude no 10 has been particularly fun as well to mess around with. Bach fugues i also find incredibly fun to mess around with and play with all sorts of articulations, i just love piano so much .
Earlier today I played the first 2 movements of the Hammerklavier (very badly) , and half of the slow movement.I don;t play much nowadays, and am getting old, and lost mental stamina finishing the slow movement.