If you wanted to see only one cathedral you might well choose Chartres. As Chartres was built within a very short period it has an unusual unity which has survived without alteration or significant restoration. It is most famous for its stained glass which dates from the 12th and 13th century but as an early gothic building it had an enormous influence on gothic art and buildings. The sculptures and the statues are some of the most important from the middle ages.
A visit to Chartres and back can be comfortably done in a day from Paris as the train journey only takes an hour.
Suggested train times:
From Paris Montparnasse to Chartres
1 hour 1 Minute