I think it's not about who is "really" the best. I think it's about who we think deserves the title: the best. So what makes people deserve to be called the best? I think this differs on people. Some think that beating everybody in casuals is fair enough to deserve the title. Others, including myself, claim that to deserve "the best" -title, the players recent tourney results and performances has to outweigh other players ones.
As for Mango, it's clear that he beats everybody in casuals. So if one thinks it's enough to deserve to be called the best, then there's no arguing. I myself by principle, don't judge casuals as proof of anything. Casuals are for training and for fun, not for proving someones level.
To prove your level, you enter tournaments. But mango has won tournaments, lots of them indeed. The question is, are his victories recent enough? In my opinion they are at this point on an edge to be not recent enough anymore. So Im not sure if mango anymore deserves to be called the best. I think that the system can only be fair if only tournament matches and results count. We cannot make exceptions on it, not even for mango.
And I also think that it's better for the competition, if only tournament matches and results count. It's also fair that way to other players. I mean, how could have PP proven in the tournament that he is better than mango? There was no way doing it in the tournament, since mango lost early by sandbagging. So imo it's not fair to PP and to other players, that even when they win everyone in the major tourneys, they wouldn't deserve to be called the best just because someone beats them in casuals. By this way, mango doesn't even give a chance for the players to show that they are really better than him. Im not saying they are, but since mango doesn't allow them to try to prove that in real competition, Im not sure mango deserves to be called the best anymore.