Dawn of the Dead would be my pick. I also have a soft spot for Night of the Creeps although the film's walking dead aren't zombies in the strictest sense (controlled by alien brain slugs).
I've got THE scariest zombie of all time guys...
the Micheal Jackson zombie from 'Thriller' and 'Return of the Living Dead 2'!
And from the way he really looks now, they weren't too far off!
Don't even talk to me about the anniversary edition, though. I think Day of the Dead is pretty underrated, but it is undermined by some poor performances.