Jonathan Strahan, currently in the process of compiling ToC for a "best of Fritz Leiber" title for Night Shade Books, lists his current favorites - longer list from which he intends to choose (but presumably the stories that made some impression on him). List is near the bottom of his article.
I haven't read many stories of Leiber - probably just about a half dozen. Among the ones I've read, there are two on his list:
- "A Pail of Air" is very good post-apocalypse survival story, & is online as text, MP3 reading, & MP3 dramatization.
- "Coming Attraction" is very famous, but I've never been able to figure out what makes it special. Its text, audio or comic book adaptation might be online (but I don't have a link handy).
- "Wanted - An Enemy" probably does belong to his "best of" list. A pulp era story with an interesting proposal to stop wars - a man is trying to convince apparently peaceful Martians to raid & loot earth...
- "Sanity": Appears to have been anthologized often enough, but I found it an ordinary tale. Premise is interesting, though: the definition of "normal" human behavior depends on the society within which the actors live.