If Nightfall is Asimov's best short story, "The Last Question" has to be a very close second. It's not as adaptable to other mediums or as action packed, but it's a fantastic story that manages to exist at an intersection of science and religion that Asimov (famously an atheist) isn't known for. When people talk about SF as the "literature of ideas," it's stories like this that come to mind (this story was examining machine learning as a concept long before it was a concept in the world in general).

I'm not sure any story sticks the landing like this one.