The Dark Crystal, easily. It created such a unique living world, that the others movies cannot touch.
Princess Bride I feel gets too much praise. It's a good movie, but too forced. And the aforementioned frame story really stinks. However the movie wouldn't be the same without. I would say the movie would be inconcieveable without it.
Neverending Story is a bit convoluted, but still excellent, and has some great visuals, if they haven't aged so well.
I actually had the worse memories of Labyrinth. The movie is much better than I remember, and I actually enjoy it much more now. The music rocks, no matter what anyone says. Chilly down with the fire gang.
My order: The Dark Crystal, Labyrinth, Princess Bride, Neverending Story.