It has to be something you actually have access to (gun, knife, trained doberman etc.), otherwise you aren't for real about the Zombie Apocalypse, and you are definitely going to be in trouble when it kicks off.
I've decided to make my Default a Taurus Judge since I plan on living on the road and need something that is easily concealed.
Fires .410 and .45 LC so I will be able to find ammo anywhere I go. I don't see .410 being a hot commodity during ZA, and am going to laugh at everyone else when the .223 and .308 dry up in the first month. Plus revolvers don't jam, and even a novice marksman could split a dude in half with a single round from this thing.