I like Regular Show more, for the lulz, but Adventure Time is cool because the heroes and villains aren't so clearly defined like most hero oriented cartoons.
They live in a post apocalyptic world with Finn being the only human remaining after some huge nuclear war. The ice king is the major "villain" in the series, but he is revealed to have been a genius who went insane and had everything he loved slowly stripped from him and desert him before becoming the monster he currently is.
It's a smart show wrapped in psychedelic imagery.