Darth Motherfucking Revan preparing to throw down with three badass Jedi, right at the beginning of the game. Bastila may not think Revan can win, and Revan may not say a word in that cutscene, but it's obvious by the way he flourishes his lightsaber that he's about to disabuse some notions. At least, until Malak shoots him.
The last line you get to say before dueling Darth Malak on the Leviathan has the potential to be a crowning moment - you can (and probably will, if you're going Light Side) say "Your power is no match for the light!" Consider it: The Reveal has just occurred - you were Darth Revan. You have every reason to flip out, because everything you know is wrong. And yet, in light of the universe lying to you, in light of discovering that you were a Dark Lord of the Sith, you can still stand fast to your beliefs and proclaim that you now stand with the light, you will never fall into darkness again. That's pretty damn inspirational...
There's an equally awesome line you can say to Malak if you've decided to go Dark Side: "I am the TRUE Dark Lord of the Sith! Now bow down before me!"
The above line actually can be used several times in the game, not just on Malak.
And, in case you want to play it nicely neutral before going either way, as Malak starts gloating about his apparent victory, you have the option of saying "You seem to have forgotten that I'm still alive, Malak!" This line reminds one of Luke in Episode V, where he cockily tells Vader that he's "full of surprises".
Malak saying that expects that all his apprentices and the droids of the Star Forge will not kill Revan, but hopefully they will slow him down.
Malak gets that with Revan in the light side ending along with a Crowning Moment of Heartwarming. Revan tells him he's sorry he led Malak to the Dark Side, Malak tells him that it's not Revan's fault, he chose to walk down that path and in the end "I am nothing".
"The apprentice has learned his final lesson."
The Dark Side ending. The main character as the returned Darth Revan sending out an effectively infinite, invulnerable fleet of Sith ships to conquer the last remnants of the Republic, led by an apprentice who will guarantee victory in any battle, all to a marching tune of Palpatine's theme. Galactic Conquest never looked so good.
If you're light-sided, the ending is just as awesome! Standing with your True Companions under a clear sky, everyone smiling, Dodonna and Vandar congratulating you. Your party waves to a cheering crowd... Good Feels Good!
Playing a "light side" character on the Sith homeworld of Korriban. After infiltrating the Sith academy and watching in bemusement as Master Uthar and his apprentice Yuthura attempt to use you as their pawn to betray each other, the player is given an ultimatum by both as to which one the player will support against the other. Answering, in effect, "I choose neither. I'm not a Sith, you idiots, I'm a Jedi." and then beating both simultaneously — and then, as frequently as not, the entire rest of the Sith Academy — ought to dispel any lingering questions about the ability of good guys to be badass. One of the most joyfully righteous and satisfying moments in RPGs.
Even better, side against Uthar and convert Yuthura back to the light side if you talked to her earlier about why she joined the Sith in the first place. She runs off to the Jedi Academy on Dantooine if it hasn't been blown up yet, leaving you to kick the rest of the academy's collective ass. And that's not even counting the opportunities to mock and sabotage the Sith's efforts earlier on the planet. Basically, Jedi!PC on Korriban demonstrates why the Sith desperately need a Detect Good force power.
Fine, let's count the opportunities to mock and sabotage the Sith before. Converting Mekel, Dustil, Kel, and Yuthura to the Light and the plain desertion of Dak. Killing off the rest of your class that you don't redeem. Facilitating the escape of defecting Sith students and a droid. Conning Uthar into giving you credit for exactly this. Redeeming effing Ajunta Pall three millennia after his death. And this is ignoring the students you kill beforehand and the fact that you wreck the entire academy top to bottom afterwards. Awesome indeed.
Hell, even your companions can get in on putting down the Sith. Of note, JoleeBindo can let out a stellar dig that doubles as one of the game's funniest bits. Having Canderous there as well gives an added bonus.
Jolee: Thank you! It takes effort to be properly irreverent at my age.
One other satisfying example is sticking it with a fellow student in the Sith Academy. From the start you know you're not going to like him: he calls you "freak" and boasts of killing you, among other things. Then along comes the tomb of Ajunta Pall, which gives you three swords, of which one belongs to Ajunta Pall himself, and you have to figure out which is the real one. You're about to exit when this student comes along and tries to bully you into handing over the real sword, intending to get the credit of finding it for himself. Naturally, you can kill him, but that's not as satisfying as giving him one of the fake swords and returning to the Academy to see Master Uthar Force-choking the student to death for failing to verify the sword's identity. And you get no Dark Side points from it! Sure, you do lose the sword, but there are better weapons out there.
On the Star Forge when Malak sends his entire army to attack you. This is an awesome moment because he admits that he isn't sending them at you out of any hope that they'll kill you, because he knows they don't stand a chance against you. He's only doing it to buy time for him to prepare for the final showdown.
A PC that has embraced the Dark Side deep enough can actually beat Jorak on Korriban at his own little game by giving the right answer to a trick question he technically has no way of knowing.
Neither. A true Sith never dies.
Choosing Mission for the Leviathan escape. She is a fourteen-year-old Twi'lek girl, sporting no special abilities other than a talent for survival. She's got the lowest hit points of your party. She's surrounded by Sith and stripped to her underwear. But she still mouths off to the guard, picks his pocket, then fights and/or sneaks her way through the deck of a Sith warship crawling with trained enemy soldiers to save a pair of Jedi and a Republic war hero. That, friends, is one Badass teenager!
How about choosing Canderous instead? The first time playing, this troper didn't realize that the equipment was right next to him in a bin, so Canderous took down two armored sword-wielding sentries bare-knuckled and wearing nothing but a pair of boxers.