Now at least poison has two things weak to it, with the fairy type.I will have to say Normal or Poison.
Normal because..NO super-effective attacks !! (not counting Arceus ofc )
Poison because it has only one thing's weak to it, and it's not very effective against Fighting, Poison, Rock, Ghost, and unnefective against Steel.
The other type could be Bug, but i think the Bug type is rising again with pokemons suck Galvantula , Scolipede , VOLCARONA !!