The Canterbury Tales as a whole don't have just one protagonist, since they're a whole collection of different stories, with different protagonists and antagonists for each one. With the Prologue and frame story, though, it's possible to think of Chaucer as the protagonist, since it's through his eyes that we see all of the action. However, if we think of a protagonist as someone whose motivations and concerns drive the plot, then Chaucer is not one, since his role is really that of an impartial narrator or observer. Defined this way, many different characters have their time in the spotlight. These occur most often in their prologues, when it's their concerns and motivations that are central to the action.