I thought I would contribute to this thread too.  Favourite composers
are definitely easier to define than greatest (if that in itself is
meaningful).  I'm sure everyone of us have vastly different list of
favourite composers, and I would like to see why each of us prefer a
composer over another and or like the composer at all!  Of course, for me,
my opinion is formed by what I have heard, and I'm still trying to expand
my listening repertoire.  I'm still fairly young, at 21.

1. Vaughan Williams
He has always been my favourite, especially since I was introduced to his
music through his English Folksong Suite, The Lark Ascending, Fantasia on
a Theme by Thomas Tallis and Fantasia on Greensleeves.  I also love his
various symphonies, especially the mysterious Ninth and the violent Sixth.
Very individual and distinctive style, which probably explains my
preference for him.

2. Stravinksy
He has written a lot of music in differing styles.  Very innovative,
daring and distinctive.  Apart from his famous trio of ballets, he has
also written stuff like the Ebony Concerto, Symphonies of Wind Instruments
and the Fairy's Kiss, works of very different styles.

3. Debussy
Imaginative orchestrator.  Sublime harmonies, in my opinion.  Dreamy works,
and full of colour!

4. Ravel
Also a great orchestrator.  His works are full of clarity and precision.
Very colourful.

5. Nielsen
His symphonies are great! Expansive ideas.

6. Adams
Minimalistic neo-romanticist? Interesting works, and memorable ones like
the Violin Concerto and Chairman Dances.

7. Sibelius
Symphonies, tone poems.

8. Copland
Americana.  His 'modern' works like Piano Variations, Connotations.  Two
very different aspects.

9. Rautavaara
Beautiful soundworld.  Great orchestration.

10. Hovhaness
Very individual, and tuneful.  Interesting orchestration and techniques.
Beautiful solo lines over haunting harmonies.  Works like Mysterious
Mountain, Celestial Gate, and the Prelude and Quadruple Fugue.

11. Torke
Modern and energetic.  Very wonderful rhythms and spiky sounds.  I like his
Javelin and his Four Proverbs.

12. Prokofiev
Humourous.  Spiky harmonies.  His symphonies are great.

Regards,
Edwin Phua
Singapore