It helps to see Heavy Metal in a theater at midnight with the volume cranked to 12 with the smell of marijuana (or Plutonium Nyborg) in the air. At least, that's how I first saw it.
I wish they hadn't botched up Heavy Metal 2000 so badly because I'd love to see a follow-up in the same vain. I know there was some effort to do so but I think it got bogged down in development hell.
Looking at The Fifth Element, I'm pretty sure Luc Besson must have been inspired by it.