Sorry to give a clich answer but I like The Road Warrior the best.
Max is at his coolest, I love the Gyro Captain, the action scenes and the photography. It's trim, mean and lean with a bigger budget but still has the look and feel of a raw, Ozploitation flick.
That said, I can understand why people like the original so much. For one thing it remains the only unique Mad Max movie because Thunderdome and Fury Road have just continued with the Road Warrior world and copied it's basic plotline and character arc for Max (helps survivors, sets them free, regains some humanity by the end), which means the original is unlike any other in the series.
It's also the most downbeat of the entire franchise. There's no saving the oppressed, or regaining some humanity, he gets the bad guys but he's lost his friend and family and become the very thing he told Fifi he feared he would - "a terminal crazy".

I like that it's so conducive to a soundtrack, but there's only one obscure song in it. I know that George Miller didn't have the budget to include a bunch of rock songs or whatever, but other than Nightrider referencing an AC/DC song, it's not dated by a bunch of late-70's music.
The score is incredible on its own, making Mad Max even more timeless.

Other than the most exciting car chase ever, this is fairly mediocre, which is of course, part of it's charm.
"He's dusted, busted and disgusted, but he's ok"