At the end of the Prologue, after we have met all the pilgrims that are heading off to Canterbury, a contest is proposed. The idea is that each of the travellers will tell two tales on the trip out and two on the way back. Of all of those stories, the Host will then judge the winner of the story telling contest by determining which story "is best told, / That is to say who gives the fullest measure / Of good morality and general pleasure."
That means that the best story will be the one that had the best balance between being a fun, clever, interesting tale that also has a very important and substantial moral. As you read the tales you can see how a tale like the Miller's Tale has a lot of fun, but not a very important moral. You can conversely see how the Parson gives a very moral, but very boring sermon. Tales such as the Pardoner's seem to display a better balance. The tale is suspensful and clever, but has an important moral about greed. Chaucer didn't complete the full set of tales that is mentioned in the the Prologue, and he never has the Host declare a winner; that is something that readers can debate for themselves.