In my opinion Mad Max was a master piece of exploitation cinema and the most realistic of the bunch.
The Road Warrior pushed it a bit further and was at least plausible. I think it's the best of the series and one of my favorite movies of all time.
Beyond the Thunderdome is tough. They destroyed the Interceptor early and Miller seems to not understand the importance of a car as a character. I think reusing the actor from The Road Warrior in a different roll was a confusing mistake and some parts were pushed to the edge of silly. On the other hand, the homage to Lord of the Flies was beautifully done and somewhat made up for the other deficiencies.
Fury Road was a completely implausible comic book that delved into the silly to a ridiculous degree. But even worse, it was not a Mad Max movie other than it copied The Road Warrior in much the same way the 7-9 star wars sequels copied Star Wars.
Edit: I will also say that after seeing the film I was shocked it was Miller. I assumed it had been passed off to some hack and was made purely for the benjamins.