Depends what kind of zombie. If they're the slow zombies (like Resident Evil 1-C:V) then I'd go with something powerful and accurate for headshots. Probably either a Lee-Enfield or some kind of battle rifle, such as an L1A1 SLR (British version of the FN FAL) or M14. If they're fast zombies (L4D, etc) I'd go with an assault rifle. Most likely an AK-47, as one of those will always work, even if its been buried in mud for several months.