Al Pacino utterly discombobulating Hank Azaria's character (and Hank Azaria himself, as this was an ad-lib) with:
Vincent: 'Cause she's got a...GREAT ASS!! And you've got your head all the way up it!
The famous coffee scene. Not only is it a Crowning Moment for the movie and cinema in a meta sense (as it's the first time Al Pacino and Robert De Niro ever shared the screen in the same scene in a movie), but also for the characters in the movie; in a film which isn't without its bombastic and grand moments of awesome, it's strangely fitting that its most compelling moment should be a simple, quiet scene where two protagonists share a coffee, have a pleasant conversation and discovering that, despite the fact that they work on polar opposite sides of the law, they're two similar guyswho actually kind of like each other.
People hunt down the eatery just to sit in that booth.