In order of least favorite to favorite:
Episode I: The Phantom Menace
Episode II: Attack of the Clones
Episode VI: Return of the Jedi
Episode III: Revenge of the Sith
Episode IV: A New Hope
Episode V: The Empire Strikes Back
Although, I and II are pretty close for me.
Phantom featured some very good action sequences (pod racing and Darth Maul fight), but the plot was a bit too simple, and until the end of the movie, nothing much of importance happens. It felt more like "we're here, now we go here, then there"---just actions, getting from Point A to Point B to Point C. However, Anakin and Padme's romance in
Clones was a joke, even though I liked the action sequences (again, this time the arena battle and the opening Clone War battle that followed) and Obi-Wan's solo mission involving Jango Fett. I like
Empire best for all of the reasons already mentioned, but mostly because it tells a story that matters. Yoda and Luke's training is about growing up and learning to be an adult, Vader's story is about a father trying to reconnect with his son (in an unconventional way, but still
, and Han and Leia's budding romance was convincing because it happened slowly and fit the characters' personalities. I also love the foreshadowing in
New Hope, even if it was unintended (Beru: "He has too much of his father in him", Owen: "That's what I'm afraid of"). Plus, the character motivations were believable, and the story flowed nicely. And while
Revenge of The Sith wasn't perfect, I liked the Obi-Wan/Anakin dynamic, as well as the Palpatine/Anakin dynamic. The contrasts between those two opposing relationships were sharp and--for me--provided a convincing enough reason for Anakin to turn to the Dark Side. Padme should have been a much stronger character, though, and was ultimately a waste of Natalie Portman's acting talent. But I did like her line about tyranny being greeted with applause.