It depends on what you mean by greatest singer. Eminem is a rapper, Jay-Z says "Yeah" about every other word, and Katy Perry is an alien.
I think that in terms of vocal range it has to be Aerosmith's Steven Tyler. Celine dion is also very good. There's also Bruno Mars. I Think that Bruno Mars is the best, he has such a boss voice, and he NEVER autotunes.
That made me laugh because its so true! in terms of best im not shure I think sick puppies singer is really good, Matt Tuck, David Draiman, M Shadows, Nathan Biggs and Jared Leto of 30 seconds to mars is really good. my fav might have to be Adam Gontier.