Gengar would also be awesome. Since it's a ghost-type it can't be attacked by the Zombies. It can also walk through walls and clear out buildings with it's Psychic abilities.
I think I would go for Alakazam. It's the smartest non-legendary pokémon :)
It would have the ability called Magic Guard, which stops it from getting poisened(turned into a zombie). The moves that I would teach it is:
Thief - (Steal the possible items that the Zombies are holding)
Teleport -...