Dylan, easily. Extremely under-rated singer, he's raw and natural and perfect for his music. This chatter about the hack Dion as a technically adept singer is irrelevant to anything connected with music that conveys genuine emotions. She's an automaton.
Tom Waits is excellent but in small doses.
I don't consider Dylan underrated. Dylan is one of those singers who can't sing but has such amazing songwriting talent that he changed the landscape of music and the way we classify " Vocals ". It's OK to have a gravelly old voice and to half talk/half sing in a song, thanks to Dylan. I consider him to be a Legendary talent..........I'm just not a huge fan of his voice...lol.
Of course I'm not a fan of Celine, but aesthetically her voice is easier on the ears.