While I feel that Empire is technically better in some ways (Irvin Kershner was a master at his craft), the original Star Wars will always be my favorite. It's the one that started it all, and like we heard already: it stands perfectly well on its own.
Star Wars was the result of a series of miracles that turned into one of the greatest films ever made through a combination of stubborn will, luck, and friends in the right places. It's a miracle that the film was made at all. And had George Lucas been able to make exactly the film he wanted to make, with no limitations and no studio pressure, the film might not have been half as good as what we got.
Intense heat and pressure turns carbon into a diamond. Star Wars was that diamond, while the prequels were simply lumps of coal.
--SKot