Pirates generally preferred smaller, more agile ships such as sloops. Really a pirate was looking for something fast that could still overpower a merchantship such as a Fluyte and was quicker than the larger ships used by the navies of the world.
Sloops had several advantages to the pirate in that they had a shallow draft (the shallower the draft, the less water a ship needs to operate in), could outrun pursuit and still could carry upwards of 20 guns.
Brigantines were another ship often used by pirates, a smaller two masted ship that carried around a dozen guns.
Galleys were used by Mediterranean Pirates, but they use primarily use oars and are not typically suited to open ocean.