I am just going ahead and assuming you mean the most beautiful piece for us individually. I have always loved the Rondo Capriccioso, Op. 14. The introductory section is quite lovely, and often that beautiful theme of the rondo has gotten stuck in my head! If we're counting more than just his solo pieces, the 2nd Piano Concerto is one of my favorite works ever written for piano and orchestra.
Topic: Mendelssohn