One-Winged Angel. Just the intro, before the Latin. Specifically this version.
The best final boss music to a video game was 29 years ago and has not been topped since. Of course, with the rise of PlayStation and disc-based games (ironically, where Final Fantasy 7 was released and in which form), games became more about the journey than that final fight. There were still "last boss fights," but they became less the focus and more about the end of the journey. But after beating Sephiroth for the first time, and he sprouts a wing (just the one!) and this song kicks in, and the real fight begins... that's when you knew you were in some shit. Especially if you reached max level and Sephiroth took literally an hour to beat (I think this was an early anti-cheat thing).
Singing this in Rockband was fun as hell. Especially if you had the lyrics to Weird Al's version ("Pretty Fly for a Rabbi") handy and sang those lyrics instead.