Before I say anything, I'd implore anyone that enjoys anything to do with the prequels to watch these asap
http://redlettermedia.com/plinkett/star-wars/
I've had some difficulty ranking them lately but as it stands,
3. Return of the Jedi
2. Empire Strikes Back
1. A New Hope
A New Hope is my favorite because as a stand alone movie, it's essentially perfect. We have clarity on everything, the main character, the main villan(s), a reason to like Luke, a reason to hate Vader and the Empire, a supporting cast that means something, a clear plot, a reason to care about the outcome and the story itself is so simple yet so effective due to how well it was executed. This movie deserves a lot of praise, it's a classic. If I was to look at the originals as one movie and the titles as parts, Part 2 (Empire) is the best because it flipped the switch with the advantage going to the Empire, thus adding new dimensions to every character. As a result, Return of the Jedi is now host to a complete cast, every character has been through good and bad and we got to see them push back and destroy the empire. Plus, in Empire, we learn a ton of new and important info about the characters that mattered, which made for an entertaining viewing.
As for the prequels, I'll let Mr. Plinkett explain.
Fuck those movies.