An impossible question it is true, but if a gun was pressed to my head (a situation that would doubtless please many) I would say B's Diabelli Variations is the greatest piece ever for piano. I believe Brendel to name but one thinks the same.How can you choose one?! I like Beethovens Sonatas, all of Chopin, Debussy Arabesques, Mozart Sonatas and fantasia in D minor and Bach Prelude and Fugues
Buy a double volume of Beethoven's Sonatas cheap on Amazon. You'll thank me.Mostly i like mozarts piano sonata and duos, beethovens moonlight sonata, rachmaninoffs prelude in c sharp minor and right now debussy's clair de lune.