I heart Okiayu-san, though the first seiyuu to make a big impression on me was actually Koyasu Takehito, and my all-times favorites are mostly women who do boy voices. Also, you haven't lived 'til you've heard Okiayu as Jinnai in El-Hazard. (First series, though I'm no longer sure how to tell which one that is.)

Inui/Tezuka for the win, glasses.