Shizuka Itou. The astounding range of her roles can be viewed
here.
Among her most proficiently known roles are
Katsura Hinagiku (Hayate no Gotoku),
Wilhelmina Carmel (Shakugan no Shana),
Morishima Haruka (Amagami SS),
Hasekura Rei (Maria-sama ga Miteru),
Takei Hisa (Saki),
Kousaka Tamaki (To Heart),
Kanzaki Kaori (Index/Railgun),
Kunogi Himawari (xxxHOLIC).
Her vocal talent is outstanding, and I've seen her in most of her roles since
Tohno Akiha in the non-existant Tsukhime anime adaptation. Regardless, her ability to adapt her voice to a variety of different characters in different situations, whether as a main or supporting has never ceased to impress me.