In Gengarry Glen RossAlec Baldwin practically steals the entire movie (which is all great) with a single 7 minute scene which has become one of the most quoted, iconic scenes of the last 20 years.
What are some other single scene performances which have stolen movies (or television shows), or become memorable or iconic in the same way?
Christopher Walken in Pulp Fiction.
Not up to that level, but there’s Vincent D’Onofrio’s turn as Orson Welles in Ed Wood.
Howard Beale’s “I’m mad as hell” speech in Network. Peter Finch won an Academy Award for the role shortly after he passed away.
Ned Beatty in Network
Teri Garr in The Conversation (technically two scenes, but I think it follows the spirit)
Sterling Hayden in 9 to 5
Wilford Brimley in Absence of Malice.
Robert Shaw’s story in Jaws. Watch the other actors. They are amazed.
Also, Rutger Hauer dying in Blade Runner. Forgettable role for the most part, but the man knows how to die well.
The China Syndrome and The Client, also
Ned Beatty, as Arthur Jensen, a chairman of a powerful financial conglomerate, one-ups Beale’s “I’m mad as hell” speech with a monologue about the primal forces of the global economy, I think.
EDIT: With apologies to** KneadtoKnow**, whose post I see immediately after I post, of course.
Great minds, Tarwater. Nothing wrong with that. 
And sticking to Network there’s Beatrice Straight’s performance as William Holden’s wife finding out about his affair with Faye Dunaway. Well deserved Oscar win for that one scene. I think it was her only scene.
Gordon Gekko from Wall Street made his “greed is good” speech, which became fairly well known.
Although you’re asking for single scene performance and all the previous posts have been single actor things, I would suggest that this scene and these two actors be considered. Most of the ones I would have named before this one have already been named, but I submit True Romance Sicilians Scene
The “I’m funny? Funny how?” scene from Goodfellas
The fax destruction scene from Office Space
I was going to mention the “Greed is Good” scene from Wall Street, but Precambrianmollusc beat me to it.
Excellent scenes, but not what the OP is looking for, I think. These are scenes that steal the movie, but all of the characters are major characters in the movie.
The OP is asking about characters that only appear in a single scene yet steal the movie. Alec Baldwin is only in one seen of Glengarry Glenn Ross, yet is by far the most memorable part (and, IIRC, it was a scene written for Baldwin, for the movie).
They hardly steal the movie given the quality of the overall thing, but there’s several very memorable single-scene performances in The Princesss Bride. Offhand: Billy Crystal’s Miracle Max, the deaf priest at the climactic wedding, and the albino’s good-natured torture-prep.
That’s the easy one.
How about the restaurant scene?
I think I’ve watched most of the movie in youtube clips, but I can’t find the part where Karen introduces all the mob wives. It’s one of my favorite movies and the whole thing is filled with great scenes. I think I’m partial to it because so much of it hits home. Not that I’m in a mob family, but I was brought up in an Italian community. I was raised around lots of people that have the same names (Tony, Joe, Sam, Rose, Mary, Rosemary, Roseanne, Anne, more Tonys, more Joes, More Marys etc…), I’ve spent my life (to this day) entering restaurants through the kitchen door instead of the front door. I’ve never flushed coke, but I’ve met people who’ve had to skip town because they’ve owed the mob money. I’ve never had a gun pointed at my face, but I’ve known some mob people (my dad has stood in front of a grand jury over it, we’ve lost business because we’ve been pushed out by them).
ETA, I think I may have missed the point as well.
In that case, we’ll go for the obvious. When most people think of Goodfellas, they think of Joe Pesci’s Funny How? scene. Though I think that scene has taken on a life of it’s own in that I think more people ‘know’ it then have actually seen it.
He wasn’t just in a single scene, but Jack Nicholson’s “You can’t handle the truth!” monologue from A Few Good Men.
jim carrey cold-cocking his boss with a coffee percolator in batman 3, “Caffeine will killya!”
To clarify, I was asking about instances where an actor only appeared in a single scene - Christopher Walken in Pulp Fiction, Ned Neatty in Network and Dennis Hopper in True Romance were the kind of examples I was talking about.
I’d also accept examples where an actor really only had one big scene, though, or maybe only a couple of scenes. Basically I’m looking for peformances that do a lot with minimal screen time (with an emphasis on the show stealing single scene moments).
In my opinion, NPH’s scene in Harold and Kumar Go To White Castle stole the movie.