..... is Attack of the Clones.
I can hear all of you screaming, but let's assess this shall we? I'll show you how I came to this conclusion.
The Phantom Menace
- establishes the core characters and their relationships
- provides an origin for the character of Anakin Skywalker/Darth Vader
- propels Anakin on his journey to Jedi Knighthood
- introduces the concept of a Jedi prophesy during which a "chosen one" will bring balance to the force
- leads you to suspect that Palpatine is manipulating things behind the scenes
- shows us the first move in Palpatine's final scheme to seize control of the senate (without the invasion of Naboo he would not be Chacellor, if he were not Chancellor there would be no Clone Wars, without the Clone Wars there would be no army, without the army Palpatine would not have overthrown the Jedi)
Attack of the Clones
- builds on relationships previously developed
- continues the story of Anakin
- shows Anakin during his journey to Knighthood
- furthers your suspicions regarding Palpatine
- shows how Palpatine's ultimate plan develops
- sets the stage for the final act of this trilogy
Revenge of the Sith
- destroys some relationships and defines others
- reveals the mechanism by which Anakin becomes Darth Vader
- confirms your suspicions about Palpatine
- illustrates how Palpatine's scheme finally allows him to achieve his goal
- reveals the mechanism through which certain Jedi are able to continue to communicate with living beings after their bodies have died
- builds a bridge between the trilogies
A New Hope
- establishes a different set of characters and the relationships between them
- illustrates Vader's character as having achieved a state of near-complete evilness
- does not deal with Palpatine's story at all but does hint at the type of governing he is using as the Emperor
The Empire Strikes Back
- reveals to Luke that he is the offspring of Darth Vader
- reveals that Vader is capable of some emotion other than cold hatred
- develops the character of Luke Skywalker into a close approximation to a Jedi Knight
- re-introduces Yoda and Palpatine after 25 years
- sets the stage for the final act of the Saga
Return of the Jedi
- reveals how the character of Anakin Skywalker/Darth Vader redeems himself, bringing balance to the force and fulfilling the ancient prophesy first discussed in Episode I
- shows how Palpatine's greatest fear (losing his power) is realized
Of course there are a million more "points" we could make under each movie's heading, but in one movie's case the verbs are different. The only purpose of AOTC is developmental. In my opinion there are no "key" characters introduced, and every key occurrence is reiterated throughout the rest of the Saga.
What I'm saying is this: if you had to watch the Saga in only 5 movies, you'd get more of the story by choosing to miss Attack of the Clones than you would if you missed any other chapter.
All that being said, I don't think AOTC is the worst movie of the lot - to me it's 4th out of 6...... but that's not the point.