My favorites are Marvin Gaye, Nick Cave, Paul McCartney, and David Gilmour (Pink Floyd).
I'll be the first to admit that I don't necessarily listen to a singer's voice for their ability to sing. I also like a lot of bad singers.
Marvin Gaye had impeccable control over his voice -- never for one moment did it ever not sound exactly how he wanted it to. Nick Cave doesn't sing so much as he intones. He phrases his words with the conviction and gravitas of a preacher running through Sinners in the hands of an angry god. McCartney is a chameleon. "Martha My Dear", "Helter Skelter" and "Back In The U.S.S.R." have no business being sung by the same person, let alone residing on the same album. Gilmour just has the most beautiful male voice I've ever heard. I don't normally care for that style of singing, but his voice has a richness and fulness to it that I just have to respect.
Those are mine, what are yours?