There's no such thing as strongest Pokemon in Pokemon Dungeon. However, if you need an Electric Pokemon, I suggest Magnemite/Pikachu (if you manage to recruit Pichu and evolve it, or direct recruit), Grass: Bulbasaur or Shroomish, and Bagon/Trapinch (Trapinch is not Dragon, but Flygon is) for Dragon.
IMO, those Pokemon are not preferable to use in Dungeon game. Good Pokemon in this game are the ones which can use room-range moves.