I would have to say

Shaun of the Dead (laughs and gore)
Cabin fever (pure gore fest)
Brain-Dead (oddball gore and laughs)
And last but not least, educate them with some cult classics:
Dawn of the Dead (Original)
Night of the Living Dead (original may bore them being black and white, so in a last ditch effort go for Savini's re-make)
Oh, I'm jus going down the zombie path-just go for the early George A stuff and you'll be fine!