I'd use the arbiter's "lemon Grenades".
That or the plasma cutter and line gun off deadspace.
Example:
Me:"oh no a horde of zombies"
*insert space-type noise of line gun as it takes their legs off*
Dismemberment is the key to a succesful zombie weapon.
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If these aren't allowed then the lever action rifle off F3

oint lookout because:
1) Theres quite a few around, so its easy to repair.
2) It has more ammo then a unlimited pistol.
3) Point. Squeeze. Head explodes.
4) It will do ridiculous damage to turrets any survivors set up as well.