My favourite is in Guards! Guards! Errol the little swamp dragon* fights the gigantic “king” dragon
and defeats it by knocking it out of the sky with the Discworld’s first sonic boom
The same book also has Vimes’ Do you feel lucky** scene, and Carrots sword in the stone trick.
** City Watch motto Fabricati Diem Pvnc
Mike Jones tackle at the end of Superbowl XXXIV.
http://upload.wikimedia.org/wikipedia/en/8/85/The_Tackle_SBXXXIV.JPG
I will be watching that a bunch this year. 
I always get choked up when I read/see the ending of A Tale of Two Cities.
The scene from The Curse of Chalion, when a young punk of a nobleman challenges the main character Cazaril to a duel. His response is to pin the young bravo helplessly to the wall and inform him that :
I could not believe it took 70 posts to get to the “Adama Maneuver”:
That episode has not left my DVR.
Along similar lines, the EAS Agamemenon emerging from the explosion of the final Global Orbital Defense platform over Earth on Babylon 5.
That settles it I will be renting the season DVDs…
I liked Firefly - Serenity River vs the Reavers scene too
I nominate the scene prior to that when they jump to the planet with the Reavers on their tail
[seeing a whole fleet of Reaver ships coming at him]
The Operative: Target the Reavers. Target the Reavers! Target everyone! Somebody fire!
Boo beating the crap out of Randall in Monsters Inc.
The “let me tell you a story about the beginning” sequence form the last episode of Cosmos.
When showing the DNA molecule as it unwinds Sagan intones:
“Among these natural experiments a molecule arose that was capable of making crude copies of itself, slowly, imperceptibly life had begun.”
To the music of Vangelis, shivers.
“It says one hundred percent guaranteed, moron!”
“Mister, if you don’t shut up I’m gonna kick one hundred percent of YOUR ASS!”
The moment in Buffy season 3 when the whole graduating class throws off their gowns and level weapons at the Mayor.
In Lynn Flewelling’s Tamir Triad, at the end of Hidden Warrior, when Lhel
allows Tamir’s demon brother to take her life, as she took his.
There’re a bunch more. Got to think about it.
The whole audience broke out in applause at one point in Live Free or Die Hard (an overall execrable movie, BTW), when the BBG gets frustrated with trying to talk to Bruce Willis on his walkie-talkie & hands it to his character’s kidnapped college-age daughter. He’s hoping for some emotional blackmail, maybe to strike some fear into the character’s heart as he listens to his teenage daughter cry & beg to be saved. This does not work out as intended from his point of view, when she starts feeding him intel instead.
“I can’t talk to this guy … Here, you try. See if you can get him to focus.” (Bad guy hands the walkie-talkie to the girl.)
“Dad?”
“I hear you, Lucy.”
" … There are only five of them left." (Bad guy grabs back the walkie-talkie.)
You get so used to the ‘helpless captured female’ trope, it can be quite entertaining when the HCF doesn’t follow the script. 
(Oh, and this was shortly followed up by the comic line of the movie, when Willis’ temporary partner meets the daughter. “I know that attitude, I’m just not used to hearing it from someone with hair.” I totally recommend watching just that five minutes of the movie.)
Roy Batty’s “tears in rain” speech from Blade Runner. Not only the CMA for that character but also for Rutger Hauer as an actor.
Agreed.
I still love “You just took out a HELICOPTER with a CAR?!” “Well, I was out of bullets.”
For all that season 6 was subpar (OK, it kinda sucked), that was pure excellence.
Omar Little, stick up boy, schooling a defense attorney in court on The Wire. Best moment of TV ever IMO.
When Inigo Montoya gets to say his line to his father’s murderer in The Princess Bride.
The final 12 minutes of The Fountain.
I’m going to inject a little girlness in amongst the comics, B5, and action movies! Although it would take me the better part of the night to sum up the awesomeness that is Diana Gabaldon’s Outalnder and the series of books that follow, I will limit myself to one scene in the latest book, A Breath of Snow and Ashes. Claire’s rescue from Hodgepile’s gang. It was all kinds of awesome. It was the kind of awesome that made me speak aloud while reading it and say YES! Take that you bastards!
Yes! Roger’s drum talking through the forest and then later…
If you want condensed Jamie awesomeness, try www.lallybroch.com and look up “Jamie-isms” if you haven’t already. I know just how you feel about not being able to pick just one yelling-out-loud moment in the series!