"What is your favorite piece played in Your lie in April?" From @cabbageandcats

In the original soundtrack by Masaru Yokoyama? If that's the case, then Yuujin A-kun no Watashi no Bansousha in Ninmei Shimasu.

If you mean songs in the anime, then Kirameki by Wacci.

But if you mean the ones played by Kaori and Kousei (which is probably what you mean), then Kreutzer, Winter Wind, or Moonlight. Hey, there are a lot of amazing pieces! I can't just pick one! Idk if this counts but I also love Clair de Lune, which played when Tsubaki and Kousei were in the classroom together.