The entrance of the Executor. We see it casting shadows on Star Destroyers! LIKE THAT BEHEMOTH FROM THE INTRO OF THE PREVIOUS FILM!
See also Visual Effects of Awesome, if only to reiterate the above (which is not hurt at all by the scene being the debut of The Imperial freaking March). A good contender however is a later scene which provides a rear view of the Executor and shows that even its engine exhausts dwarf the nearby Star Destroyers.
To truly put this into perspective, normal Star Destroyers are approximately 1,600 meters long, or about 1 mile. Super Star Destroyers are 12 times longer than normal Star Destroyers. The Executor is a ship over half the length of Manhattan!
During the battle on Hoth, Luke single-handedly takes down an Imperial Walker, by himself, on foot. At that, you'll understand why the Emperor is so concerned about the kid; as in, "If the boy can do that with barely a few days instruction from Obi-Wan Kenobi, what could he do once he is properly trained?"
The Walkers themselves.
During the same battle, the Rebels manage to disable a Star Destroyer with the Ion Cannons, creating a gap in the Imperial blockade for the Rebel ships to escape through.
After the Battle of Hoth, the Millenium Falcon is making its escape from not one, not two, but three Star Destroyers. Outmaneuvring the Star Destroyers with a twisting dive, the colossal ships can't readjust their positions in time, and so we're treated to the sight of two of the three Star Destroyers colliding in space. It's awesome.
Luke informs Yoda that he's just not strong enough in the Force to raise his starfighter from the swamp. Yoda then raises the X-wing out of the swamp, accompanied by triumphant music and spirited beeping from R2-D2.
Luke: I don't— I don't believe it!
Yoda:That... is why you fail.
"Judge me by my size, do you?"
"For my ally is the Force, and a powerful ally it is. Life creates it, makes it grow. Its energy surrounds us and binds us. Luminous beings are we, not this crude matter. You must feel the Force around you; here, between you, me, the tree, the rock, everywhere, yes. Even between the land and the ship."
Yoda has tons of cool lines. Another good one is "Do, or do not. There is no try."
Some of Yoda's lines have made it onto fortune cookies, as "Do or do not, there is no try" has been found along with other Yoda lines.
Luke Skywalker VS Darth Vader.
Above all, when Luke is clinging to that ledge, thoroughly beaten by Vader both physically and emotionally. Luke willingly defies Vader's offer - "Come with me, it is the only way." - and steps off the ledge. Not even knowing if he would survive the fall (he does, barely).
With that move, it has been interpreted that meant Luke won that fight. That is because Vader was not out to defeat Luke, but to corrupt him to turning to the Dark Side of the Force; when Luke showed that he would rather die first and mean it, Vader learned that his son's integrity was too strong for him alone.
For Vader, when he starts picking up random pieces of machinery and tossing them at Luke in the middle of the fight. That's when you really understand that, yeah, Luke's pretty handy with a lightsaber, but he iswayout of his league here.
Compounded by the realization that Vader isn't even trying to kill Luke. He wants him alive but beaten to within an inch of his life, so as to be vulnerable to his suggestion to join forces. Imagine how brief it would have been if Vader had just wanted Luke dead.
Further emphasized that while Luke adopts the standard two-handed grip on his lightsaber at the start of the match, Vader opens with a more casual one-handed grip and bats away Luke's initial attacks with what is clearly contemptuous ease. Those first few strikes serve as the first inkling that Luke may have just bitten off a lot more than he can chew.
The fact that, despite the handicap Luke faced, he actually managed to land a blow on Vader's shoulder, though the armor stopped it.
The setting for this particular duel was also amazing. The eerie, low-lighting of the carbonite chamber, the fog from the machines, and Vader's breathing grimly cutting in through the silence. A truly masterful set-up to the fight.
When you think about it, Luke actually makes a decent showing, especially for his low level of training. When Vader begins the fight, he's just barely even trying. Then Luke evades his trap, and even manages to cause him some degree of pain with the coolant (if his cry of rage is any indication), and Vader resorts to throwing things at Luke using The Force. Then Luke lands a hit on Vader's shoulder, causing Vader to cry out in pain AGAIN. Only when he chops off Luke's hand is Luke finally driven to his knees. Sure, he'd probably have lost eventually anyway, but considering how far below his combat abilities are below Vader's, that's pretty impressive.
Just about everything Vader does through this fight is awesome, when you realize that he was controlling both the pace and tone of that battle the entire time. He starts off by forcing Luke to make the first overt attack (which, by the way, is in direct contradiction to the Jedi code Yoda had just taught him, making it a nice Take That at the same time), then he almost playfully bats Luke around a little to show him what he's up against. In the second encounter, he does nothing but stand to one side and pummel Luke with Force thrown objects, showing him what a real adept is capable of, before knocking him onto the suspended walkway. And then, he goes full bore berserk on the boy, and Luke switches from mildly out-of-his-depth action hero to full on terrified and desperate to be anywhere else. And Vader lays him out in about twenty seconds. And what does he do when Luke gives him some flack after all that? He takes off his fricken sword hand at the wrist with almost no effort at all. Only then does he start delivering his true Breaking Speech. Pure Bad Ass.
Also, when Vader kills Admiral Ozzel. Ozzel isn't even in the same room, and yet he easily force chokes him through the view screen; distance is no object to Vader.note Although more to the point, distance is no object to the power of The Force.
When the Millenium Falcon's hyperdrive finally works thanks to R2.
Wedge and Wes being the only Rebels to actually take down an AT-AT with their tow cables.
Boba Fett doesn't even flinch when Darth Vader gets in his face about the disintegrations.
How about the fact that Boba is able to get semi-huffy with the Sith Lord and live!
Really, everything Han did in the asteroid field.
Han Solo after being tortured. When he finds out that this was all to lure Luke into a trap, he uses a bit of strength he has left to punch Lando before being restrained. Lando was in the middle of a Heel–Face Turn at the time, but the fact that Han's best friend putting his other best friend in danger being the last straw for him was pretty awesome.
As Han Solo is lowered into the carbonite freezing chamber, Leia declares her love to him. She doesn't get the response she was hoping for.
Leia: I love you!
Han: I know.
Especially since this line was an ad-lib by Harrison Ford.
"I am altering the deal. Pray I don't alter it any further."
"All too easy."
"Impressive... most impressive."
The big reveal, in some of James Earl Jones's best vocal work.
Vader: No. I am your father.
Darth Vader just force deflecting Han's blaster shots with his hand, no lightsaber necessary, then Force-ripping his blaster across the room. You think you're going to be able to just shoot Darth Vader? Please!
Imagine yourself as Han Solo for a moment. You've just outrun a bunch of Imperials and finally made your way to a small out of the way planet for a chance to rest and repair your ship. You've met up with an old friend of yours and are on the way to a nice meal with the possibility of romancing Leia a bit afterwards. Then you open the door to find none other than Darth Vader waiting for you. Do you look in disbelief to the friend who betrayed you? Turn and hightail it out of there with Leia and Chewie in tow? No. Without a single second of hesitation you immediately pull out your gun and start shooting that motherfucker. Further proof that Han is the biggest badass to ever swagger across the galaxy; too bad Vader has more than enough power with the Dark Side to make up for that.
Then there's Chewbacca's truly epic roar. It's a real shame he didn't do that more often in the series.
John Williams' music is its usual awesomeness, but two aspects of this score particularly stand out. Firstly, the Imperial March, which makes its debut in this film; rarely has music been quite so badass. The second is at the very end of the film: The Rebel fleet has fled the galaxy after the destruction of their main base, Luke is psychologically shaken by the confrontation with Vader and learning the man is his father (not to mention getting his hand lopped off) and Han has been frozen in a block of carbonite and taken off by a notorious bounty hunter to be sold to wicked gangster. But as Lando speeds off to rescue his friend and Luke and Leia stare out at the stars, what music is blaring behind them all? Han and Leia's Love Theme.
This was also Needa'sCrowning Moment of Awesome. Having just been outmaneuvered and out-bluffed by Solo, Needa is tasked with explaining his failure to Vader, and there's no question in anyone's mind how Lord Vader deals with setbacks. What does he do? He flies himself to the Executor to stand before Vader, take full responsibility, and fall on his sword, sparing his ship and crew from the Sith Lord's wrath. Vader's quoted line seems to imply that even he was mildly impressed by Needa's actions. Compare Needa's execution to Ozzel's, whom Vader pointedly refused to acknowledge while choking him to death.
With Luke and Han both incapacitated in different ways, Leia calmly takes charge of two rescue missions, in a manner similar to her staying back at Echo Base on Hoth to direct the evacuation until the very last minute.