I can't possibly pinpoint one scene, but here's a few of my favorites:
The first 45 minutes of "Full Metal Jacket." R. Lee Ermey is great in that one. I couldn't even tell you what happens in the rest of the movie. I watched it all the way through one time, but now I don't remember it.
The town meeting scene in "Hoosiers" where they're voting on whether to fire Coach Dale. After voting to fire the coach, Jimmy Chitwood walks in and says, "I play, coach stays. He goes, I go." Leading to a re-vote, and the tearing up of the ballot followed by, "Coach stays."
And somebody already mentioned "Big Trouble in Little China." This one is filled with gems. However, my favorite is when Kurt Russell throws his knife (and misses badly) at Lo Pan. Lo Pan says,"Good knife. Goodbye, Mr. Burton," and throws the knife back at Russell, who manages to catch it, and immediately throw it back at Lo Pan (this time stabbing him square in the forehead and killing him). The leads to all the statues in the room crashing in a domino effect. Kim Cattrall looks at Russell, bewildered at how he did it. Russell (full of himself) responds, "It's all in the reflexes." Classic.