I have to say Placido Il Mar from Mozart's opera, "Idomeneo"

or

Eric Whitacre's "Sleep"

or

Der Gang zum Leibchen by Brahms...

hard to pick one... but I'll go with the Brahms, I think.

Mussorgsky - Dawn over the river moscow
stravinsky - rites of spring
rachmaninoff - rhapsody on a theme by paganini

there are others i know by tune, but not by name :(

Pretty much all of Holst's "The Planets"!

Ditto ^

Incredible music and a very interesting man.

It's not the most complex piece of music, but my favourite would be Romance Without Words - Faure. It's the epitome of French Romanticism ^_^

JS Bach - Concerto for Harpsichord in D minor, the first Allegro.
Franz Lizst - Hungarian Rhapsody No. 2 and No. 6
Debussy - The Engulfed Cathedral, and Minstrels Prelude

there are so many, but Three Romances by Schumann is at the top of the list

easily Danse Macabre by Saint-Saens

where does one start?

Rachmaninov (more specifically) variation 18
Grieg's Piano concerto in A min
Liszt's Hungarian Rhapsody #2
Bartok - Concerto for orchestra - intermezzo interrotto
Holst - The planets (jupiter)
Mussorgsky - Pictures at an exhibition
Chopin - Fantasie Impromptu
Orff - Carmina Burana (unbelievable live!)
Tchaikovsky - 1812 Overture

I don't want to stop listing them there but I will to spare you all.

^That guy there has taste.