It's the original, the template. The source code for an entire genre. That's enough.
I'm going to observe that there are very few Europeans on this forum. I'm less surprised that another non-American would rate, or even say they have read Hammett, than if an American said it, despite there being hardly any "foreigners" regularly on here. Dickens is the ultimate trash writer of English Literature; selling his stuff by the episode. He wrote in disgusted terms about the state of England at the height of its powers; about work houses, the treatment of the poor and the hypocrisy of the rich. He wrote adventure stories. So why would there be reverence for him, over a native author who wrote in a similar way about what lies under the rock of American society? Fitzgerald is a better simile writer and Hemingway is more epic, but elevating them over Hammett is pure snobbery. "The Great Gatsby" degenerates into a cheap crime story...Great American Novel, though.