You can't go wrong with a shotgun! However, The most fun i ever had killing zombies is in half life 2 ,we dont go to ravenholm or whatever it is cold, with the gravity gun and the saw lades or barrels! that was the best stage i have ever played!
i would personally go with the sniper crowbar cause its quite practical for sighting distant hordes and opening useful supply crates. so my melee weapon = sniper crowbar. for my other weapon i would prefer to use dual wield tactical shotguns. XD