Megumi Ogata-Haruka: So much emotion and strength. She makes the most dramatic scenes tear jerkers [spoiler]remember in S when she shot her own heart crystal out
Yuuko Minaguchi-Hotaru: She was just amazing as Saturn and Mistress 9. As Saturn, she sounded so small, yet so powerful.
Keiko Han-Luna/Beryl: I prefer her voice for Beryl. She made her sound so young and pretty (unlike the dub which made her sound like an old hag).
Akira Ishida, Toshio Furukawa, Ryôtarô Okiayu-Fish Eye, Hawk's Eye, Tiger's Eye: The Trio are one of my favorite villians, and their seiyuu make them sound funny and creepy at the same time.
Blarg, their's too many seiyuu for me to list so I'll just go onto the Dub voices
Tracey Moore-Serena: So cute and young. Pretty much what everyone else has said.
Sabrina Grdevich-Pluto (R season):Okay, so she sounds a bit like a valley girl in real life, but she is an amazing actress, and I love her Pluto in R. I don't know why the voice director made her sound stupid in the movies.
Dub Mimette (Don't know her voice): I didn't like her Japanese voice. I thought her dub voice fit her better.
And although I didn't like her voice for Serena, I feel that an honourary mention should be given to Terri Hawkes. She is an amazing actress in live action shows, and I think that she should have done Luna's voice, but of course the voice caster was too drunk to notice how perfect she would have been. Oh well.