It's hard to define "best" since there are so many different poker games and variations and formats, plus the difference between online and live but i say Phil Ivey, Unclear how well he does now but he was the most feared player in the world for a solid decade.
Very difficult to say, because I haven't seen good players from the past play. Among everyone I've been able to follow, Phil Ivey is the one I like most due to his style of play.