Al Pacino givin 'em hell at the end of Scent of a Woman.
From “Pulp Fiction” the scene near the end when Jules disarms Tim Roth’s character in the restaurant and proceeds to explain to him why he isn’t blowing his head off (“Reach in there and grab my wallet.” “Which one is it?” “It’s the one that says BAD MOTHERF*CKER!”)
A couple of scenes from near the end of “Unforgiven” - the one right after The Schofield Kid has killed Quick Mike at the ranch and he and Will Munny are talking while Will Munny keeps an eye on the prostitute riding her horse towards them (“It’s a helluva thing killin’ a man. You take away all he’s got, and all he’s ever gonna have.” “Yeah. Well I guess they had it comin’” “We all have it comin’, Kid.”).
Then the one following that when Will Munny decides to ride into town to take care of Sherriff “Little” Bill Daggett in the only way he knows how to. (“Well sir you are a cowardly sonuvabitch. You just shot an unarmed man!” “Well he shoulda armed himself if he’s gonna decorate his saloon with my friend.” “You’d be Will Munny outta Missouri. Killer of women and children.” “That’s right. I’ve killed women and children. I’ve killed just about everything that walked or crawled at one time or another. And I’m here to kill you Little Bill, for what you did to Ned.”)
Neo choosing to face down Agent Smith in the subway tunnel towards the end of The Matrix.
For funniest, mine is a tv scene: Pushing Daisies, at the funeral home when a re-animated corpse is locked in a casket where Ned can’t touch them, meaning in 60 seconds someone else will die. Panic ensues.
Crocodile Dundee: “That’s not a knife. Now, THIS is a knife.”
If you lived in New York City back then, this scene was wish-fulfillment by proxy. It was Bernie Goetz without getting sued for it.
There’s a scene early in The Hunt for Red October, in which the sub is moving very quickly down an undersea canyon, hiding from another sub that’s hunting for it. The canyon is very short, so it’s important to turn before they crash into the wall of the canyon. One of the officers is freaking out, saying, “Captain, we must turn! Captain!” Ramius (the captain, played by Sean Connery) is calmly making the calculations in his head (you can see his lips move as he’s counting the amount of time they have). He barks an order to relieve the officer who was freaking out and then orders the turn at the last moment. Basically, the scene neatly establishes him as a badass and the best damn sub captain in the Soviet navy.
FInally, one I wholeheartedly disagree with. To me, that whole movie was Pacino at his hammiest.
My favorite Pacino scene was his diner conversation with DeNiro in ***Heat. ***
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The ring dropping scene in The Sixth Sense
The first time we see the creature’s entire body in Alien: Dallas is searching the service ducts with a flame thrower, and Lambert is monitoring his presence on radar. She suddenly sees another red dot converging on Dallas and tries to warn him. He turns around and sees the creature for half a second as it hisses and prepares to attack. The audience jumps, and next scene has Parker finding only the flame thrower.
The other scene from that movie, where Emma Thompson’s character breaks down on finding that Hugh Grant’s character is not married after all. What precedes it in the movie does make it much more meaningful, but it also stands on its own as a wonderful piece of acting. Here’s a clip: https://www.youtube.com/watch?v=88_tyulOQDE
Can’t believe this one’s still on the board.
Blade Runner - Roy Batty dying.
The scene in Casablanca where Victor Laslow has the band play “La Marseillaise” and Louie shuts down Rick’s café.
Jack Nicholson in The Last Detail, just before the brawl in the bathroom where a Marine tells him: “It must be a pain in the ass to undo all those buttons just to take a piss.” Nicholson: “Yeah, a Marine just needs to take off his hat.”
That and "I AM the motherfucking shore patrol, motherfucker! I AM the motherfucking shore patrol! Give this man a beer! Meadows: “I don’t want a beer.” Budduskey: “You’re gonna have a fuckin’ beer!”
The “We don’t have to show you no steenking badges” scene from Treasure of the Sierra Madre.
Emptying the shitter in Christmas Vacation.
Paul Newman in nearly every scene of Nobody’s Fool. His finest movie, IMO.
I’ll add a comedy one from the Albert Brooks’ “Lost in America”:
Hilarious all on its own. 
Madeleine Kahn’s “Woof!” when she sees the monster’s monster schwanze for the first time.
Madeleine Kahn’s song a la Marlene Dietrich in Blazing Saddles. Puts me on the floor every time.
Danny DeVito in Throw Mama From the Train:
Owen: He’s my cousin, Momma.
Owen’s Mama: Bullshit! You don’t have any cousins!
Owen ::smacks Billy Crystal with a hot frying pan:: You lied to me!
My all time favorite: the scenes in Zulu! when the Zulu warriors are singing their war song, pounding on their shields and charging into gunfire. Beautiful and terrifying at the same time.
How could I possibly have forgotten that one?! (“You want ANSWERS?!?” “I think I’m entitled to them.” “You want ANSWERS?!?” “I want the TRUTH!” “YOU CAN’T HANDLE THE TRUTH!!”)
For me it’s that whole last bit after he lets Verbal go and then sits down to drink his coffee at that other guy’s (the one played by Dan Hedaya) desk after insisting to Verbal Kint that “Dean Keaton WAS Keyser Soze!!” And then when he drops his cup and sees the name on the bottom of it after it breaks…D’oh!! (“The cripple, did you see him?” “The cripple, where did he go?!?”)
A more recent one for me is from the climactic scene in “Flight” (with Denzel Washington) when “Whip” Whittaker is at the inquest and has to ask the inquisitor to repeat herself over and over again when she asks him if it’s his opinion that the young airline employee who’d tried to save the little boy’s life in the plane crash was the one who had drunk the unaccounted-for bottles of vodka. I thought Denzel was masterful in that role. I think in almost any other year he would’ve won Best Actor for his performance in that film.
From Godfather, Part II:
The rooftop and hallway stalking and murder of Don Fanucci by Vito Corleone during the festa in Little Italy.
Untouchables - the train station stairs shootout
ST-IV:TVH - Chekov being interrogated. “I do not know your name! …[snip]… May I go now?”
The audition scene in Walk the Line. Can’t find the video online, but here’s the song. I love how he starts off uncertain, then gets stronger and stronger as the band picks up the tune, and the record company guy starts digging it. A neat bit of acting just in the way he performs a song.