Title says it all. List the one that will remain in your heart until you wither and die of old age.
For me, there are 10 trillion that I love, ranging across every single game, but nothing can wrest The Red Wings from its perch atop my favorite.
Maybe it was the classic airship intro, watching Cecil bitch those mobs with White and Red fangs, or maybe it was watching his troops slaughter innocents, or maybe hearing the lightning crack when first entering the Lunar Subterranean as everyone looked at Cecil and he nodded like a boss, but the song just says everything I need to know about FF as a whole.
Epic.