> Ghost vs Zombie
> Ghost kills Zombie
> Zombie comes back as a Ghost
> Ghosts can't be beaten. It just creates more ghosts

This is going to be a tough one. I like both of them equally, so there's no immediate choice…

…going to go with Zombies, simply because they're the closest thing to skeletons. Gengars and Boos aren't enough to tempt me, plus it means I can do this:

NYEH HEH HEH!