Roger may have been the Pirate King, but for me the greatest pirate that has ever existed in the manga has been Whitebeard. I don't care about the issue of his power level, his achievements, or whether he's far from looking like a real-life pirate. I say this because I have read on various sides that the character was weak, dumb, and loser.
His personality, influence and leadership to me are unmatched. He was a true big boss in the world of piracy. Despite the fact that he suffered in childhood for being an orphan, he knew how to get ahead with courage and effort. For many, the dream of having a family may seem stupid and simple because of the kind of pirate he was, but I consider that having that dream wasn't an impediment to having power and influence. He empathized with his crewmates and allies, and despite being an elite pirate, he was a person who had humility and knew how to recognize the good things in others. He lived and died with honor.
And yes, my opinion is probably biased, but I think Whitebeard was a great character. I wish there would ever be a spin-off of him.