(in alphabetical order)
Amadeus
Arsenic and Old Lace
Back to the Future
A Christmas Story
Groundhog Day
Mary Poppins
Men in Black
Monty Python and the Holy Grail
The Princess Bride
Raiders of the Lost Ark
Shrek
Star Wars (episode 4)
Strictly Ballroom
For what it's worth, here are the ones mentioned by others in this thread so far that I would add to my own list of perfect movies.
The Empire Strikes Back
Ferris Bueller's Day Off
The Godfather
Indiana Jones and the Last Crusade
The Matrix
Raising Arizona
The Sting
Toy Story
Toy Story 2
What About Bob?