I still think "The Mule", AKA the second half of "Foundation and Empire", could make an excellent standalone - still one of the best sci-fi thrillers IMHO, and

I still think "The Mule", AKA the second half of "Foundation and Empire", could make an excellent standalone - still one of the best sci-fi thrillers IMHO, and much more interesting characters than most of Asimov. But otherwise, there are so many better choices for translating to screen...