(answer to the question "If you were to compile a list of "must see" films that an alien should see when visiting earth, what would you suggest as a top five?")
I guess 2 movies would do. These two answer the 2 key questions any alien would ask :
1) is there intelligent life out there ? => "Airplane!"
2) who's in charge down here ? => (my all time fave) : "Dr. Strangelove or: how I learned to stop worrying and love the Bomb"