Ultimately the pistol you have in your hand is best. As an experienced undead extermination philosophizer, I would implore you to chose a firearm with a rail to mount a hand gun bayonet. As any experienced Zombie fight tactitian knows, you will eventually run dry on hot lead. Better have sharp steel available.
Pistol Bayonet - CQB Handgun Knife - Ka-Bar Laserlyte
Pistol Bayonet - CQB Handgun Knife - Ka-Bar Laserlyte