I think the best would be the classic Zombie Vs Shark scene from Zombie Flesh Eaters.
In fact, I liked it so much that i got it tattooed on me! Excuse the crappy photo, it's really hard to take a photo of it because it's in an awkward position.
As for the worst, I'd have to say the entirety of an atrocious film called Children of the Living Dead. Not even Tom Savini and a zombie wearing wing tips could save it.
EDIT: AAAAND after reading through the thread properly i've realised I'm not the only one who hated Children of the Living Dead!!!