Return of the Living Dead (1985)

Directed by Dan O'Bannon

Perfect mixture of gory horror and comedy. There's some really funny parts but there's also some really terrifying bits too. There's a fantastic punk soundtrack by the likes of the Cramps and TSOL. 

These zombies are a result of a military chemical called Trioxine, which is released by two haphazard medical supply warehousemen. To cover up their mistake they take the resulting reanimated animals and such across the way to a friendly mortician to dispose of them in his incinerator. That's where it starts. The smoke from the incinerator permeates the clouds releasing a toxic rain onto the nearby cemetery.