Maybe Francis Drake, who basically plundered the whole of the Spanish Main, and then, for kicks, took the long way home, becoming the first commander ever to sail around the world. (Magellan died mid course).
Maybe Captain Morgan - who not only plundered treasure ports, but became Lieutenant Governor of Jamaica after being pardoned by the crown --- and had rum named after him?
Maybe Captain Teach, aka Blackbeard, who was certainly feared above all others. He was an utter madman, with a psychopathic disregard for human decency, and a habit of burning fuses uner his hat while he fought?
But all of these would be wrong.
The greatest pirate who ever lived was a Chinese matron named Cheng I Sao. She commanded a fleet of over 500 ships of 36 cannons (making them frigate-sized by Western standards) and 80,000 pirates. In 1805, this made her navy significantly larger than that of the United States Navy, for instance. She married Cheng I, a man who was a third generation pirate, and when he died after being blown overboard in a gale, she effortlessly took over the already sizeable fleet. She also married the strapping young lieutenant that her husband had initially taken prisoner and then come to love like a son - making him her consort and co-pirate royalty. At the height of their power, they boasted they could have toppled the Imperial government. Whether this was a truthful boast or not historians may debate - but it's certainly true that the coast of Southern China was ruled by her, and the local economy was determined more by her whim than anything else.
Finally when Western powers allied to defeat her (having tried and failed many times initially) she decided on the simplest solution, and offered to become the Imperial Navy. The Emperor readily agreed, and paid her for the privilege, as well. Little is known about her retirement, but it is thought that she opened a public house and lived to the age of 69. (Her husband took a comission in the Army and died in battle.)
I can't tell you how delighted I was to read this story.
(Because lots of people have been curious, I'm throwing in some links. Some of them are dubiously accurate, but then - almost everything about pirates is dubiously accurate...)