When I was a kid I was scared of the Emperor when he is torturing Luke with force lightning. The second half of Revenge of the Sith is also pretty gut-wrenching, especially the Jedi being killed, Anakin bursting into flames, and the birth of the twins juxtaposed against the introduction of Darth Vader.
Probably the Force lightning scene in ROTJ. Other than that I can't remember a moment in a Star Wars movie that scared me.
when the mynock suprises leia
when darth vader appear in that hole in dagobah, it freked me out the first time i saw it
Seeing Anakin about to kill the younglings, and seeing Vader's face in RotJ.
For me it would have to be when Darth vader walks in the cave on dagobah. I actually thought vader was on dagobah about to kill Luke the first time i saw it.
The All-Time SCARIEST moment in SW:
"Mesa Jar-Jar Binks, Mesa your Humble Servant"
That would easily be the last half of Revenge of the Sith
Like when,
Chancellor Palpantine transforms into Darth Sidious and then kills Mance Windu
When Anakin kills all the young padawans
When Obi Wan almost kills Anakin (I teared up when Obi Wan starts crying)
When Padme dies while giving birth to Luke and Leia
Anakin becomes Darth Vader
And Darth Sidious' creepy laugh at everything