My personal favorites are:
Mad Max: Thunderdome: I thought the first two were great too. The characters in Thunderdome might have been over the top, but they're easily some of the most memorable ones ever. Plus it made me jealous of Ike Turner, which obviously took a lot of excellent storytelling.
Night of the Comet: Night of the Comet...bad acting, poor plot, ridiculous method for murdering the world...Why do I like this movie? Kelly Maroney, and Valley Girls with Uzis. Genius.
The Quiet Earth: Best apocalypse ever. If only the secondary actors weren't so bad it might be on my list of best movies ever. It easily gets the most kudos for having a complex, well-thought out series of action/consequence of any "the world ends" movie I can think of. I would love to see someone capable redoing this film.
And...Just for giggles since we're also allowing zombie movies:
Shaun of the Dead: What's not to like?