Oldschoolways1 argues that the existence of Hot Fuzz prevents a vote for Shaun of the Dead... and yet recommends a vote for Zodiac in spite of the existence of Fight Club and Se7en.
I would agree that Shaun of the Dead is not Edgar Wright's best movie, but it is his most influential. A love letter to the zombie genre and the inspiration for a return to horror/zombie comedies spawning numerous inferior knockoffs.
Zodiac, by comparison, did not generally capture the imagination like his hit film Se7en or his cult hit Fight Club. I generally only ever hear praise for Zodiac from aspiring filmmakers. As a piece of art/entertainment I personally find Zodiac very slow and plodding, so I presume it must be cinematography and shot composition that the film buffs find so impressive.
Magnolia is the crazy film with the rain of frogs where all the actors randomly burst into song, right? You want to pick THAT over one of the most influential comedy movies and most influential zombie movies of that decade?