What Is Ultimate Zombie Killing Weapon?

Probably a trained Tiger (assuming animals can't be infected)
Or an armoured vehicle of some variety (like a sealed train).

Cream pies

I'm going to have to go with a nuke.

Lawnmower, it works in Dead Rising.Marka1700

Zombies and shotguns have always just gone with each other.

People with a good level of common sense. And sufficient combined arms. Because there is no way in hell a Zombie Apocalypse should be sizable enough to actually doom the world. Lest of course the damn thing is airborne.

Gets your point across no matter what situation. I like to dub it "The Justice Gun". This also applies to a combat shotgun.