CT Maybe the most defining moment?

For me the best defining moment is after Luke defeats Vader in ROTJ, throws down his Lightsaber and says to the Emperor:

"Never. I'll never turn to the Darkside. You've failed, your Highness. I am a Jedi, like my father before me."

After two full movies of Vader and the Emperor hunting down Luke to turn him to the Darkside, their failure finally comes to realization at that moment.

As far as R5 breaking down in ANH, I like to think it was just chance. Besides, clever little R2 probably would have figured a way to escape from the Sandcrawler on his own, and went out and found Ben. Although it was the next morning that the Empire slaughtered the Jawas looking for him, so he would not have had much time. Plus, it would have been difficult for him to find a way to get the restraining bolt off. He wouldn't have been able to trick the Jawas the way he tricked Luke into taking it off.

 

Hah I've watched all of them so much that I really only watch each film 2 or 3 times a year now. Right now I'm in Tolkien-mania mode because of the last Hobbit movie lol