Eriol's seiyuu (or voice actor) is Nozomu Sasaki. I think he did a terrific job portraying Eriol. Here's a list of all his seiyuu works (taken from Anime News Network):
.hack//SIGN (TV) as Bear's Son
After War Gundam X (TV) as Olba Frost
Akira (movie) as Tetsuo Shima
Angel Cop (OAV) as Asra
Ashita he Free Kick (TV) as Rovelt
Asobotto Senki Goku (TV) as Jou
B'tX (TV) as Koutarou
B'TX Neo (OAV) as Koutarou
Black Jack (OAV) as Jun (ep. 9)
Blood+ (TV) as Carl
Blue Sonnet (OAV) as Wataru
Bubblegum Crash (OAV) as Mackie Stingray (Ep 3)
Bubblegum Crisis (OAV) as Mackie Stingray
Cardcaptor Sakura (TV) as Eriol Hiiragizawa
Cardcaptor Sakura Movie 2: The Sealed Card as Eriol Hiiragizawa
Compiler (OAV) as Igarashi Toshi
Darkside Blues (movie) as Katari
Doraemon: Ganbare! Gian!! (movie) as Moteo
Doraemon: Nobita's Adventure in Clockwork City (movie) as Pib
Doteraman (TV)
Dragon Fist (OAV) as Fei Long
Earthian (OAV) as Chihaya
Ehrgeiz (TV) as Hal
Fist of the North Star 2 (TV) as Tao
Fly High (TV) as Uchida
Gakusaver (OAV) as Kengo Sasaki
Gall Force: New Era (OAV) as Nova Universe
Gambalist! Shun (TV) as Uchida
Genji Tsuushin Agedama (TV) as Agedama
Glass Mask (TV 2005) as Hijiri
Goal FH (TV) as Hikaru Tatsumura
Gundam Evolve (OAV) as Hathaway Noah (Evolve 5)
Guyver: Out of Control (movie) as Friend A
Here is Greenwood (OAV) as Kazuya Hasukawa
Hi no Tori (TV) as Leona (Revival Chapter)
His and Her Circumstances (TV) as Takefumi Tonami
Inu Yasha (TV) as Shinosuke (ep 131-132)
Knight Hunters (TV) as Nagi Naoe
Knight Hunters Eternity (TV) as Nagi
Kyo Kara Maoh! (TV) as Daikenja (ep. 68)
Leave it to Kero! Theatrical Version (movie) as Eriol Hiiragizawa
Legend of Basara (TV) as Asagi
Legend of the Galactic Heroes (OAV) as Julian Minci
Legend of the Galactic Heroes: My Conquest is the Sea of Stars (movie) as Julian Minci
Locke the Superman: New World Battle Team (OAV) as Lan
Machine Robo: The Running Battlehackers (TV) as Zen Ogawa
Madlax (TV) as Baggis
Magic Knight Rayearth (TV) as Clef [note: yet another CLAMP character!]
Magic Knight Rayearth 2 (TV) as Clef
Mahou no Angel Sweet Mint (TV) as Plum
Megazone 23 Part III (OAV) as Bud
Mobile Suit Gundam ZZ (TV) as Lunan
Mobile Suit Gundam: Char's Counterattack (movie) as Hathaway Noah
Mobile Suit SD Gundam Mk II (OAV) as Hathaway Noah
Mobile Suit SD Gundam Mk II (OAV) as Hathaway Noah
Mobile Suit SD Gundam's Counterattack (movie) as Shinzaku (SD Sengokuden)
Monster (TV) as Johan Liebert
Monster Rancher (TV) as Ham
Naruto (TV) as Hayate Gakkou
Odenkun (TV) as Vienna-kun
Please Save My Earth (OAV) as Sakaguchi Hajime
Rayearth OAV as Clef
Revolutionary Girl Utena (TV) as Ruka Tsuchiya
Ronin Warriors (TV) as Shin Mouri
Ronin Warriors OAV as Shin Mouri
Rurouni Kenshin OAVs as Enishi Yukishiro
Rurouni Kenshin: The Motion Picture as Gentatsu Takatsuki
Samurai Champloo (TV) as Yukimaru (ep 16-17)
Sequence (OAV) as Toshiaki/Seran
Shinesman (OAV) as Sugura Sasaki
Suikoden Demon Century (OAV) as Miyuki Mamiya
Super Deformed Double Feature (OAV) as Mackie
Tennis no Ouji-sama (TV) as Jin Akutsu
Tennis no Ouji-sama: Atobe Kara no Okurimono (movie) as Jin Akutsu
The Heroic Legend of Arslan (OAV) as Eram
The Samurai (OAV) as Tamahara
Top wo Nerae 2! (OAV) as Gouya Reicy
Ushio & Tora (OAV) as Ushio Aotsuki
Ushio & Tora: Comically Deformed Theater (OAV) as Ushio Aotsuki
Weiß Kreuz (TV) as Nagi Naoe
Weiß Kreuz Glühen (TV) as Nagi
Yu Yu Hakusho (movie 2) as Yuusuke Urameshi
Yu Yu Hakusho: Eizou Hakusho (OAV) as Yuusuke Urameshi
Yu Yu Hakusho: Eizou Hakusho II (OAV) as Yuusuke Urameshi
Yu Yu Hakusho (TV) as Yuusuke Urameshi
Yu Yu Hakusho: The Movie as Yuusuke Urameshi
Yu-Gi-Oh! (TV) as Priest Shada (Part 7: The Pharaoh's Memory); Shaadii (series 2)