That depends: Are the zombies already coming down my street? Or are they relatively close but still not a problem atm?
If they're close, my 12 ga winchester 1300. Got a couple hundred rounds for it, so im not an easy target. Add the ammo to 100 yard, close to MOA accuracy, and its a zombie killing machine.
If the zombies are somewhat close, either run to Gander Mountain and pick up the bolt action .50 they have there, or knock over the nearest gun shop that stocks ar-15s and ammo. Both firearms offer very good accuracy out to around 500yds for the ar and well over 2000yds for the .50 (didnt check, but i think the longest confirmed kill was made by a Canadian at around 2500 yds)
Also, what the hell is a water cannon gonna do to zombies???