If we're talking Japanese voice actors, then I'd have to say my absolute favorite is Mayumi Tanaka, who voices Kuririn & Yajirobe in Dragon Ball Z, Monkey D. Luffy in One Piece, and Koenma in Yu Yu Hakusho, as well as many other various roles. I just love the sound of her voice, and whenever I hear her as Kuririn, Luffy, or Koenma, I can't help but smile.
I've also taking I liking to Ikue Ohtani lately. I've been watching a lot of Japanese Detective Conan episodes lately, and I enjoy hearing her as Mitsuhiko (Mitch in the dub). She also voices Zatch in the Japanese version of Zatch Bell.... oh yeah, Pikachu too.
If we're talking dub actors, then I'd have to say Alison Retzloff, R. Bruce Elliott, Laura Bailey, Rachel Lillis, Veronica Taylor, and John Burgmeier top my list. You CANNOT find better voices for Conan & Richard Moore than Alison and R. Bruce. You just can't. Their acting is superb and to make it better, Alison doesn't sound like your generic anime kid, and R. Bruce doesn't sound like the generic anime adult. Laura Bailey & John Burgmeier both have very good voice ranges, and I think they're some of FUNi's best actors right now. Rachel Lillis is near perfect at any character she voices, and she really brought a certain charm to Misty & Jessie in Pokemon. Plus, she has an excellent voice range. Veronica Taylor may have not been the perfect voice for Ash, which is mostly due to the feminine voice she gave him, but I've always liked the way she said her lines, and you can tell by her performances that she really got into her characters when she was acting.