My favorite would be Larry Niven’s “Lucifer’s Hammer”.
Also, an honorable mention to Tom Godwin’s “The Survivors” which isn’t a post-apocalyptic story in the traditional sense but deals with many of the same issues you would find in any end of the world story. (ie surviving the initial hardships, forming a society, etc.) Just a terrific story that seems to have slipped through the cracks.
Wikipedia has an exhaustive list of post-apoc fiction in literature and movies.
(Inspired by Gerry’s post on Earth Abides)