What are the films that are least rented by the customers?
film refers to title; least rented refers to count(min(customer_id))
SELECT T.title FROM ( SELECT T3.title, COUNT(T1.customer_id) AS num FROM rental AS T1 INNER JOIN inventory AS T2 ON T1.inventory_id = T2.inventory_id INNER JOIN film AS T3 ON T2.film_id = T3.film_id GROUP BY T3.title ) AS T ORDER BY T.num DESC LIMIT 1