No matter how dirty the Rat did him by taking a dump on his character and actions in the OT and generally ripping his legacy to shreds, Luke will forever be my childhood hero, and his confrontation against Vader and the Emperor in "Return of the Jedi" will forever be, at leats to me, his apex, although the young farmer blowing up the Death Star is right behind.
Seeing him as the REAL last of the Jedi, all clad in black and with a brand new personal lightsaber, try and succeed to redeem his father through sheer force of will and faith in the Dark Lord of the Sith's last faint glimmer of good, hidden deep inside that hate-fueled and consumed by the Dark side mechanical body, was definitely a high point. That, and a bit of tapping into the Dark Side to overpower his opponent.
And right after that feat, Luke being faithful (and ballsy) enough to metaphorically spit in the face of Palpatine, basically evil incarnate, by throwing his lightsaber away and refusing the Emperor's threatening offer is another high point.
And that's because of such heroic deeds that the Force will be with him. Always.
The interplay between Luke, Vader, and Palpatine in the throne room scene is still amazing to this day.
Luke's not betting everything on redeeming Vader. He has reasons grounded in reality for making the decisions he does throughout the film. He knows both Vader and the Emperor want to turn and make him an apprentice. It's the primary leverage he has throughout the encounter.
At first, he wanted to distract Vader away from the strike team. Then he wants to goad the Emperor into remaining on Endor. He really thought he'd be able to keep the Emperor occupied long enough for the fleet to arrive.
Everything goes sour though, with Palpatine taunting him for his apparent failure.
Luke attempts to strike Palpatine down and Vader of all people defends him. Luke doesn't want to fall to the dark side, but he needs Vader and Palpatine to think he will. Vader needs to convince Luke that he is fallen and that Luke will follow, hence him defending Palpatine. Palpatine has won, Vader can't escape the Dark Side's grasp, and neither will Luke...but defending another, even someone evil for selfish reasons, isn't the most evil of acts. Along with that, technically Vader may have saved Luke from falling to the dark side in that moment.
Vader and Anakin had reasons to stop Luke from killing Palpatine. To show Luke that it was hopeless and that there was hope.
Luke's strong enough to weather Vader's attacks, but can't gain the upper hand. Regardless, he's buying time and continuing to distract them from the Rebellion's efforts.
It's a disaster when Luke let's it slip that he has a twin sister. More than the threat now posed to Leia, Luke himself is now expendable. Both Vader and Palpatine can now kill him without a thought and try their hand with the much more vulnerable Leia. And if Vader kills Luke, what good is still within the Sith Lord will surely wither and die with him. Vader may truly become the monster he professes to be.
