RoccondilRinon said:
TPM really has more of a four-act structure: act one ends with the escape from Naboo, act two has the podrace as its climax, act three is the Coruscant section and act four is the return to Naboo and final battle. Four acts is unusual in most media; hour-long television dramas are the only format that regularly use it. I disagree that act one of TPM is too short; a lot happens in it.
ROTS has a shortish first act, and a long second one; the most likely candidate for the end of act two is Order 66.
ESB has a clear first act; it's less clear where act two ends, but I would venture Luke's departure from Dagobah makes the most sense.
ROTJ follows the standard three acts; act one ends with the sail barge battle, and act three begins with the Rebel fleet jumping to hyperspace.
The Phantom Menace is three acts, not four. It is just badly structured. The second act begins when they meet Anakin. Act II is everything on Tatooine and back on Coruscant, Act III is the return to Naboo and battles (and the denouement is the parade). That highlights the problem with TPM; it's structured around the wrong character - Padme. The blockade of Naboo should just be a MacGuffin to get the events in motion and lead to them meeting Anakin, which it does, but it becomes the actual focus of the story and the goal of the characters. Anakin is secondary, when he should be the focus. Instead we get an entire movie wasted on his introduction, a fatal flaw. Was Star Wars wasted on Luke's introduction? Nope. Compare what we know about Anakin by the end of TPM to what we have seen of Luke by the end of ANH.
In the overall scheme of things, Obi Wan should have been the lead and Anakin the focus of Episode I, but instead Qui Gon is the lead and Padme is the focus. Qui Gon is completely superfluous, there is nothing he does that couldn't have been given to the others, and Padme is subsequently abandoned in the following movies. In Episode II the MacGuffin is protecting her from assassination, which puts all the events in motion for Obi Wan, Anakin and Padme. I'd say Act II begins with Anakin being assigned to protect Padme and them departing for Naboo. Act III begins with Anakin slaughtering the Sand People out of revenge. Much better structurally, but the execution ruins it.
In Revenge of the Sith, the beginning of Act II is Padme and Anakin when she tells him she's pregnant. The end of Act II is Anakin choosing to help Palpatine - the Padme's Ruminations scene. The tipping of his character (so horribly undermined by his immediate regret after helping kill Jules).
You're right, in Empire, the turn into Act III comes when Luke chooses to leave Dagobah and rescue his friends.
In Jedi, Act I ends as they leave Tatooine, Act II ends with Luke handing himself in to Vader. This is about Luke, not the Rebel Alliance.
It's generally accepted that Act changes come with a character decision, not the most obvious break or change in action. A reader/viewer's emotional investment is in the characters, and it is around those characters that virtually every good script is structured.