Favorite seiyuu, huh… Well I don’t have a favorite per se, but one seiyuu always catches my eye whenever I see her as part of the cast: Mai Nakahara.
I guess I’m a bit biased with her roles in CLANNAD and Symphonic Rain and Mekaku City Actors, but there’s something about her voice that’s oddly calming. Heck, I even enjoy watching Wagamama Fairy Mirmo de Pon because of it! And yes, I did enjoy her being part of Charlotte, in a small way
Ironically, I haven’t finished watching Jinrui wa Suitai Shimashita which is a series that really showcases her vocal skill… Or so I hear
edit: Oh hey I just saw from @HeliosAlpha that she voiced Rena in the Higurashi anime. Having just finished chapter 1 of the VN recently, I would be veeeeery interested to see her in that role!