The scene where Anakin walks into the room and turns on his saber as the padawan walks from behind the chair. I know that sounds pretty fucked up but it's probably the best example in Star Wars that shows how twisted and evil Vader really is.
That scene sucked because even though it's supposed to be an important event, it kind of gets glossed over, even when it gets made referenced to.
They didn't have to show them actually killing kids, but all we got as angry face Anakin and the kid saying "what are we going to do?".
They should of showed some bodies on the floor or at least a scream.
And I just watched it again, just now, and it really makes me hate Vader. Not fear him, but hate him. He has a terrible motivation. He's pathetic. I want to save my girl, so let me kill children and go on a roaring rampage of revenge. By the time of the original movies, he's an old guy decaying in a suit who's just really powerful with the force. He's a lapdog.
I'm not even talking personally as I hate him, but it kills the whole trying to redeem himself scene in Return of the Jedi. And it's kind of jarring with his portrayal in the clone wars tv series. From the movies he was always selfish and never a good person.
Also I really liked Order 66. I dig scenes where everyone is getting hunted down and it looks like there's no hope of winning.