what is the best music producer?

It definitely depends on what genre you're talking about.

Overall - I'm talking about the people who have had success producing more than one genre.
Rick Rubin
Paul Epworth
Danger Mouse
Pharell

Hip Hop:
Metro
Lex Luger
Dre
Kanye

Pop
Max Martin
Shellback
Stargate
Jeff Bhasker

There are definitely more, I just can't think of them off of the top of my head.