Which artist from the past or present do you think has the most distinctive singing voice that you've ever heard? For me, I think it's definitely Freddie Mercury. His voice is just irreplaceable in the singing world.
If you mean distinctive as in easily identified rather than trained or good, Leonard Cohen. His voice, particularly in old age, was unmistakable. I've heard it called a unique blend of gin and cigarettes.
That would be John Hiatt. Hear him one time and you'll never mistake him for another artist. He makes it work for him though because, lyrically, he's just as unique.
How about the lead singer of Fine Young Cannibals, who always sounded as if his trousers were a little too tight! He had a very distinctive voice, it's one of the few times Weird Al gave up on copying it and sung the chorus in his own range.