Based upon the context, the word 'paragon' best means "model of excellence".
The word 'paragon' is used to refer to a person that is the best at doing something. However, in this excerpt from the "General Prologue", Chaucer resorts to this word to show irony since the Pardoner is actually a selfish and deceptive man who earns money by selling false relics and indulgences. Therefore, he is the opposite to what he represents; In other words, he is not the holy man that he pretends to be. The Pardoner is a model of excellence but in the negative sense.