For me it is...
1. Bach - transcendent and still surprises me to this day with his foresight.
2. Beethoven
3. Brahms
4. Mozart
5. Tchaikovsky
6. Chopin
7. Wagner
8. Mahler
9. Bruckner
10. Dvorak
I'm a sucker for symphonies.
Honorable mentions to Mendelssohn, Rimsky Korsakov, and Mussorgsky.