It's Phil Ivey because he has more publicly documented winnings that any other poker player in history. Also, he is just a beast and a mad genius at the poker tables!
I'd say when you look at all the variations of poker and look who's highly skilled in all of them you'd have to go with Daniel Negreanu as best all around imo. Not only that but just the staying power is also a big factor. Over 20 years playing at a professional level is amazing.
I think the best poker player in the world is someone who is crushing the biggest cash games in the world, and I would bet very few people know their name.
For me, the flagship player is Daniel Negreanu, I love all of his playing qualities (the ability to read the cards of the players, and at the same time his rather eccentric behavior).
Man thats tuff to decide who the best is but phil ivey has to be on the top of the list, stats speak for there self and he has proven that he is in the elite feild above most players