Prokofiev has a few candidates:
Second concerto, plenty of places, but specially the cadenza in the first movement. I wouldn't be surprised if the hardest a piano has been hit in performance is Jorge Bolet playing this.
Pleanty of volume in the three war sonatas, perhaps the loudest moment the end of the 7th.
Third concerto, last variation of the second movement,
See, it is not only the individual notes, but the accumulation of sound. You are supposed to really bang the @*&$ out of the piano at the end of Schumann's Paganini in Carnaval, but there is not as much accumulation of sound as in the examples above. Beethoven writes il piu forte possibile in Op. 109, but that spot is not even the loudest part of that piece.