Sorry to jump on the Mozart bandwagon, but I'd have to say that he's my favorite. His music just blows me away.

I have to say Beethoven, his later works, especially the string quartets are my favorites

Rachmaninov.
All of his works are brilliant

Rachmaninoff for sure, I'm a piano guy, and that stuff is AMAZING. After that, Mozart is awesome, and Hayden is great too.

But seriously, guys and girls, Romance by Rachmaninoff is worth a listen if you're trying to vibe out, or seduce somebody? haha

I tend to go for the more modern composers. I like Arvo Part, John Williams, Micheal Nyman, Phillip Glass, Steve Reich. I am a big fan of Reich and have a fair bit of his stuff though I know he can polarize somewhat.

I also have Music Of The Spheres by Mike Oldfield and am not surprised that he headed in this direction.

In no particular order with a great example of his work:

Giuseppe Verdi - La Traviata
Robert Schumann - Piano Concerto in A minor
Antonín Dvořák - "New World" Symphony
Edward Elgar - Enigma Variations
Alexander Scriabin - Sonata No. 9 "Black Mass Sonata"

In no order...

Bedrich Smetana
Gustav Holst
Modest Petrovich Mussorgsky
Claude Debussy
Rimsky-Korsakov

I find Schumann's music very interesting. The Variations in the Name of Abegg is so fun to play. I'm playing it right now. xD
I'm also in love with Debussy. His music is beautiful. I'm currently playing his Reflections in the Water. :D

I love Bach.

Beethoven. Very tragic but unique.