I think it's an impossible question to answer. So much depends on one's own background and general tastes. Sayers was the first crime novelist I ever read. That was at a time when I thoroughly despised genre fiction (I was a very swotty teenager). The only reason I read her was because she had been an undergraduate at my Oxford college. As well as an extremely well-stocked academic library, we had a small reading room for lighter reading. It was mainly supplied with books written by members of the college and one day I ventured to read one of hers. And naturally Gaudy Night appealed most, because it was set in an Oxford women's college.
I'm sorry to say that I never managed to finish The Maltese Falcon. It bored me to tears.
I wonder what qualities we are looking for in judging a crime novel as outstanding?