It's a very hard decision. Certainly in the final list would be:
Airplane
Blazing Saddles
Planes, Trains and Automobiles
Life of Brian
Blade Runner
Alien
But if I had to pick only one then it would probably be the original Star Wars. It's not the greatest film ever. But it was the first to make me go "wow" when it originally came out, and it makes me think of happy times. Just the sounds of the light sabres, space ships, etc takes me back to my youth.