Oh, my. I have so many favorite songs, and they range across many genres as well. I don't think I could ever narrow it to just one, but I thought about your question from the point of view of being stranded on a desert island with a very limited list on my ipod, and that helped to focus it.
I would want to have Aaron Copland's "Appalachian Spring", along with selections by Beethoven ( Fifth and Ninth Symphonies, Eroica, Moonlight Sonata) Bach (Art of the Fugue), and Vivaldi (Four Seasons).
I'd want some Scott Joplin rags, and some Gershwin, and Count Basie.
On the modern side, there would have to be Pink Floyd - the Dark Side album (especially Great Gig in the Sky, which I could listen to all day), and Meddle. Something from Rick Wakeman - the King Arthur album, or Six Wives of Henry the Eighth. Also some Yes - probably the Yessongs album. And the Moody Blues are a must.