Dat Revelations theme.
Nothing else has summed up the vast majesty of the EVE universe as that theme. No crashing symphonic orchestras playing for great battles, techno-futuristic rock or synth pieces, or dark, humming melodies to hint at the danger in the distance. The Revelations Theme is a piece that for me symbolizes everything that drew me to EVE in the first place: The vastness, beauty, and complexity of the universe it is set in.
Hearing that brings me back to my newbie days of marveling at new background nebulas, examining every piece of floating space junk for detail, and thinking about the seemingly-uncountable options for ships and modules that awaited me down the line. It makes me think of times when I'd stop playing for a second just to pan my camera around in awe of this gorgeous space-universe.
I miss it.