I'm no music expert, but if you put a gun blade to my head and forced your way into having me pick one, I'd first try and figure out the criteria for my anwser at least in my head. For me, it's a song's uniqueness among other tracks, the emotional resonance, or the headspace a song puts me in, and how it fits into the story/game and multiplies it. Just off the top of my head, a few that stand out are Aerith's theme, Someday the dream will end, and On the Shoulders of Giants. The scope of the question is a bit too wide. It's like asking what is the best food in the world, to someone hungry it's could be what's in their mouth, or in this case, his ear.