Midorikawa Hikaru -- Zelgadis (Slayers), Tamahome (FY), Heero Yuy (Gundam Wing), Mikage (Utena), Schuldig (that Weiss show), Rail Claymore (Lost Universe), piles of guys in bad yaoi shows

-- and more recently Zenon in Beet the Vandel Buster, one of the good bad vampires in Blood+ (I can't remember the name), Hugh (Kiba), and the big brother in Saiunkoku Monogatari (the true Prince). God I love his voice. I worship the ground he speaks over.
Another fave?
Akira Ishida - Xelloss (Slayers), Gokudo (Gokudo), Coud (Elemental Gelade), and Hakkai (Saiyuki) among others...
Kappei Yamaguchi - Inuyasha, L, and a bunch of other fascinating characters. He's got SUCH a distinctive voice, and when they don't type-cast him as a bellowing Inuyasha type he really shines.
Tomokazu Seki -- most notably as Chichiri, but fabulous in absolutely everything he's ever done. Probably the second best voice actor I've ever encountered for range of roles he's done up reaaallly well.
and of course, as Aernath so ably stated,
Takahito Koyasu, for his roles in absolutely every anime known to fangirldom

Probably the single most recognizable voice in anime, and able to leap tall buildings in a single bound. He's done everything, and done it very very well, thank you.
But Midorikawa Hikaru is still my favorite! -- along with his English counterpart for Zel (Crispin Freeman).

"The permanent temptation of life is to confuse dreams with reality. The permanent defeat of life comes when dreams are surrendered to reality."
James A. Michener, The Drifters