It's a tough call, picking classic moments--the movie is full of them, one after the other. This one's a gem, though (some of you will notice that Peter Lorre's wonderful rant later became a signature line for "Ren and Stimpy"):
(I suppose technically that deserved a spoiler alert. Ah well.)
And you can't really appreciate the aura of cold-blooded calculation that radiates from every single character in the film without this:
Dashiell Hammett famously wrote of private eyes like his creation Sam Spade:
He wants to be a hard and shifty fellow, able to take care of himself in any situation, able to get the best of anybody he comes in contact with, whether criminal, innocent by-stander or client.