Thinking about the 1st three movies, A New Hope, Empire Strikes Back, Return of the Jedi. I think for me the most surprising twist and shock was when Darth Vader reveals himself to be Luke's father. Everyone in the theater was gasping or crying out "Oh sh*&!"

I don't know. My two surprising moments were the revelation of Vader as Anakin Skywalker in ESB; and the realization of what Palpatine had achieved by the end of AOTC.

Think of Revenge of the Sith when Anakin confronts Palpatnie. Palpatine reveals himself to be a sith

The Battle of Endor was incredibly done and is one of my favorite sub-parts of Star Wars. Also, the Battle on Geonosis is a favorite of mine, too : )