Sephiroth-sama has some excellent music in the game (Ah, I love the FFVII soundtrack~), that creepy background music which starts playing whenever he appears and the almighty one-winged angel. Here's some music from Final Fantasy VII which is related to Sephiroth.

Those Chosen by the Planet (Sephiroth's BGM)
I love this music, it's creepy, ah.. the heartbeat in the background...

Chasing the black-caped man
[I'm pretty sure] The first time you here this is in the mythril caves after seeing the Midgar Zolom that Sephiroth-sama skewered, it must be related to him, who else has such a nice black cape? >D

The birth of a god
The battle music for fighting Bizarro Sephiroth, if you listen closely you can hear part of his background music creeping in...

One Winged Angel
YEAH! One winged angel! the words are perfect (read below) I also have another slightly different version. I can't decide which one is better-

One Winged Angel version 2

Yes yes I know these are all midis, but they're good ones (in my humble opinion) ^_^; If you want to hear better quality versions of these, and listen to One-Winged Angel with singing then I suggest you buy the FFVII soundtrack. Animenation are good with shipping music CDs when they're in stock, and I've heard that Game Music Online is very reliable.

Here are the words to the (in)famous One-Winged Angel:

Estuans interius ira vehementi
Estuans interius ira vehementi

Sephiroth, Sephiroth

Estuans interius ira vehementi
Estuans interius ira vehementi

Sephiroth, Sephiroth

Sors immanis et inanis
Sors immanis et inanis

Estuans interius ira vehementi
Estuans interius ira vehementi

Sephiroth, Sephiroth

(chorus part 1)
Veni, veni, venias, ne me mori facias

(chorus part 2 ~ sung over part 1)
Glorioso, generosa

translation (I should check this with a latin speaker...):

Estuans interius ira vehementi --> Burning inside with violent anger
Sors immanis et inanis --> Fate, monstrous and empty
Veni, veni, venias, ne me mori facias --> Come, come, oh come, do not let me die
Glorioso, generosa --> Glorious, noble