The Parent Trap
Both the 1961 and 1998 versions are so much fun, but if you have to pick just one, go with the 1998 version featuring Lindsay Lohan. The theme of parents rekindling their youthful love for each other and the twins' hilarious shenanigans make this film a pleasure to watch, but you may want to discuss with your child a few concerning elements (in particular, children may be concerned about the plot device that divorced parents would make no effort to see one of their children, and clarify that while the girls' pranks are funny onscreen they would be very inappropriate in real life!).
+© Disney