Hmm.. I really like Ruki's voice from GazettE, even though I don't think it's the best... it's still my favorite though.
I haven't read through this thread, but has anyone heard of Olivia Lufkin? I think she has an amazing voice for a female singer.
Anyways, top five would be:
Ruki/ the GazettE
Olivia Lufkin
Miyavi
Amano Tsukiko
Hizumi/ D'espairs Ray