I had been meaning to post a reply in the vote thread, but since there is this thread now, I'll say it here: The Maltese Falcon wouldn't even be in the Top 10 of best crime/mystery books of the 20th century for me. Maybe Top 100.
The first one that came to mind upon seeing the thread title was "And Then There Were None". I'm not a huge Christie fan, but that is definitely her best book and the book is so expertly plotted, especially for it being "the first of its kind" and leading to the now well-known die-one-by-one genre that has transferred to films.
I used to read a lot of mystery when younger but it's not often my cup of tea anymore, for the most part. So I may have missed a lot of good ones.