Problem in Nested Query
I don't think you need a sub query here. Does this give you what you need? If so it should be more efficient.

SELECT m.title, MAX(r.stars)
FROM Movie AS m
LEFT OUTER JOIN rating AS r ON r.mID = m.mID;
GROUP BY m.title


Only way it could break that I can see is if you have two movies with the same title and different mID's.