The following section lists all seiyuu / voice actors who has voiced Sephiroth at one point or other.

Miki Shin'ichirou

Miki Shin'ichirou is easily one of my favorite seiyuu. He has a strong baritone that can range from playful and ditzy, to the somber and sexy-arrogant, as seen in a lot of characters that he has voiced so far.

Miki Shin'ichirou voices Sephiroth for the Erhgeiz Playstation game (Japanese version). Sadly enough, he did not have a Japanese voice for the Kingdom Hearts Playstation 2 game, although Miki Shin'ichirou probably would have voiced him there as well.

Some other popular characters that Miki Shin'ichirou has voiced: (series alphabetical order)

Series Character
.hack//sign Crim
Angel Sanctuary Katan
Ayashi no Ceres Mikagi
Bleach Urahara Kisuke
Full Metal Panic Kurz Weber
Fushigi Yuugi Yuuki Keisuke
Get Backers Takeru Teshimine
Gravitation Aizawa Tachi
Haru wo Dateita Katou Youji
Initial D Fujiwara Takumi / Tak
Kaikan Phrase Todou Yukifumi
Kyuuketsuki Hime Miyu / Vampire Princess Miyu Larva (TV series)
Pokemon Kojiro / James
Scrapped Princess Shannon
Tenkuu no Escaflowne / Vision of Escaflowne Allen Schezar; Amano
Tsubasa Reservoir Chronicles Touya
Yami no Matsuei / Darker Descendant Asato Tsuzuki

Morikawa Toshiyuki

Morikawa Toshiyuki has one of the best "villain" voices ever, and for good reason. He has a decidedly deep, mocking and very suave voice best suited for antagonists rather than protagonists, and never sounds out of place. It's no surprise that he voices many of the best known villains in anime. (Although it cracks me up to know he voices Ichigo Kurosaki's father in Bleach, that funny bastard XD)

Morikawa Toshiyuki voices Sephiroth in Final Fantasy: Advent Children. I believe he also voices Sephiroth in the Final Fantasy: Last Order OAV.

Some other popular characters that Morikawa Toshiyuki has voiced: (series alphabetical order)

Series Character
Gensomaden Saiyuki Homura
Haru wo Dateita Iwaki Kyousuke
Inu Yasha Naraku
Last Exile Alex Low
Peacemaker Kurogane Ichimura Ryunosuke
Speed Grapher Suitenguu Chouji
Street Fighter Ryu
Tenjou Tenge Takayanagi Mitsuomi
X (TV) Seiichirou Aoki
Yami no Matsuei Seiichirou Tatsumi
Yuu Yuu Hakusho / Poltergeist Report / Ghost Fighter Shishiwakamaru

Seiyuu Trivia

In the yaoi series Haru wo Dateita, Morikawa Toshiyuki plays one of the main characters Iwaki Kyousuke. Miki Shin'ichirou plays the other main character, Katou Youji. Katou Youji and Iwaki Kyousuke happen to be lovers. ^^;

Lance Bass

Lance Bass is a member of the boy band N'Sync, and he voices the English version of Sephiroth in the Playstation 2 game, Kingdom Hearts.

I don't really have anything to add to that one. Except maybe, EEWWWW.