Vader's introductory scene - storming a rebel ship and interrogating someone (with a one-handed Neck Lift) on the spot. It's the bearing of someone in command. Then in his next scene, he teaches Admiral Motti to respect the Force. Casual, effortless choking from across the room, with some Deadpan Snark on top; everyone else in the room, with the exception of Tarkin, is rightly scared of him.
I find your lack of faith disturbing.
The first appearance of the lightsaber.
The Falcon first goes to light speed.
Leia shooting out a grate and snapping that "Somebody has to save our skins!".
Luke and Leia swinging across a bottomless pit.
The duel between Vader and Obi-Wan.
"I've been waiting for you, Obi-Wan. We meet again at last. The circle is now complete. When I left you, I was but the learner, now I am the master."
"You can't win, Darth. If you strike me down, I shall become more powerful than you can possibly imagine."
Vader runs a successful Batman Gambit to reveal the location of the Rebel base, which involves allowing Luke and Han to stage Leia's rescue. Then he's the only one Genre Savvy enough to realize the Rebels' attack might work. Then he personally gets into the dogfight in his customized TIE fighter and blows away enemy fighters right and left. Empire would've crushed the Rebels if he was in charge.
During the Death Star battle, Luke has an enemy on his tail. Does Wedge come up behind the bad guy and blow him up? No. He comes at them head on and weaves between Luke and the TIE fighter to take his shot.
The dive toward the Death Star trench, done twice, and awesome both times.
Luke taking charge during the final Trench Run. Consider that his wingmates are Biggs (his best friend and a much more experienced pilot) and Wedge (described in the EU as being so Badass that Imperial pilots are known to wet their beds during nightmares of being chased by him), and the inexperienced farm-boy with no more experience than varmint-hunting manages to tell them what to do. Wedge even calls him "boss."
Han's big character establishing moment where he quickly and calmly kills Greedo.
"Sorry about the mess."
Commander Daine Jir criticizing Vader's decisions to his face and living to tell the tale. Expanded Universe explains that he was willing to be blunt and honest with Vader, but still carry out orders and do his job quite effectively, earning Vader's respect.
''Use the Force, Luke...let go, Luke..." "The Force is strong with this one..." "Luke, trust me..."
When the Death Star goes boom!
The award ceremony. Crosses over with Heartwarming.