Hard to choose. I always go through ever-changing periods of Beethoven, Mozart and Chopin, and yes, Bach. I can't choose between these four. They are all great to me.
My favorite depends on when you ask me. Right now I am leaving a Chopin period, and going into the Beethoven, Mozart period. Bach next week. Back to Chopin.
When I am in the Chopin period, it's hard for me to hear anyone else, I still want more of Chopin, and think that the piano is by far the greatest instrument. Then I hear a Mozart violin concerto for an hour and get convinced totally differently. I don't understand myself sometimes.
But I have a lot of joy in both the piano and violin, practicing as well as listening, and especially music by these four great composers.