a few years ago, i probalbly would have said a new hope.
Now that i'm older, i'd have to say empire because of it's darkness and subtle nuances, and is more adult.
I know that everybody will probalbly think it's blasphemy, but now that i'm older and more jaded i like empire and revenge of the sith the best. kids will always love the adventure, and the positive values of the original films including episode 1. in fact among most children today phantom menace is their favorite film.
My nephew when i showed him the first five films, wanted to watch attack of the clones and the phantom menace over and over, the original films not so much.
if you look at both trilogies, without whining about how much the prequels suck, you can see the common threads between the films. Luke and Anakin are the same archtypal character they just chose different paths, but they have the same weaknesses common to all men the darkness within, and their yearning and spirit of adventure is the same. if you read the novelizations for all six films , cotinuity errors aside it makes more sense than the films, especially if your familiar with the radio dramas and have read the hero with a thousand faces by joseph campbell.