J.S. Bach is number 1.
2 and 3 are Beethoven and Schubert or Schubert and Beethoven. Listen to Schubert's Lieder and piano sonatas then explain to me how he is not deserving of this ranking.
4 and 5 are Brahms and Haydn or Haydn and Brahms.
You can fill out a top10 with Mozart, Tchaikovsky, Mendelssohn, Schutz, and Mahler in whatever order suits you.
I'd develop a separate list of post romantics. Call me crazy, but Benjamin Britten might be at the top of that list for me. Exquisitely crafted music.