Melee or ranged? What's the situation?
If it's just for killing, my choices are:
Melee: likely a flanged mace. Gives you reasonable reach, there's no blade to dull so maintenance is less of an issue, doesn't require significant skill or strength, has enough weight to crush skulls, but at the same time, striking power is magnified by the points of the flanges. If you miss the head and hit the body, a mace still has enough power to crush bone, disabling whatever you hit, with far less risk of splattering fluids everywhere because nothing is being cut up.
Ranged: High powered sniper rifle. The WWZ book falls into the Hollywood myth of silencers, in reality, they really don't do much to silence a weapon. A silenced gun is still hella loud, and won't do much to deter more zeds from finding you. And since you only want killing power, and if resources are not considered, my choice is a high powered rifle, .50cal. Headshots will obliterate the target, misses will still annihilate any limbs that are hit, any body shots that go through the spine will incapacitate the zed, giving you more time.