Off the top of my head, I have two: Porco Rosso, cause it's a fantastic film, that always leaves me filled with hope and a massive grin. Its also partly cause it's the film that helped me discover Studio Ghibli for the first time - the way in which I did was lovely too.
My then-GF had left for work early one beautiful Saturday morning, leaving me half asleep sat on the couch with sun blazing through gaps in the curtains, and I was flicking through channels in an attempt to wake up. I got to Film4 and this film had just started, this gorgeous animation, so I figured I'd give it a shot, and by the end of it, I had fallen in love.
The lovely relaxed atmosphere, the warmth, and the wee columns of light coupled with an absolutely fantastic film made it an experience to remember. The massive bag of cheddar and onion Kettle Chips may have helped too... :$
The second is Jurassic Park, because DINOSAURS*
*(Ok, there's more to it that that, but I waffled enough about one film, two and everyone will run away
)