Holst's Planets Suite
Partly because my Dad played it often, partly because Holst grew up near where I did, partly because it kicked off my teenage education in music, I will always have a special place in my heart for The Planets.
You should absolutely listen to it now, particularly Jupiter, the Bringer of Jolity, and Mars, the Bringer of War (my favourites by a long way). My music teacher got us to close our eyes in our first class with him, played Mars, and asked us to describe what we thought was happening. I'll never forget the David Attenborough-esque march of the red ants I saw so vividly, clashing against the black ants as that piece sways and moves. (Perhaps I played too much SimAnt that year?)