I think it would probably be Conkeldurr, Alakazam, Gengar, Tyranitar, Salamence, and Hydreigon would be one of the toughest in my opinion. Conkeldurr for it's massive strength, Alakazam for the brains/psychic power, Gengar for his ability to become people's shadow and phaze through walls, Tyranitar for it's capability to destroy mountains, Salamence because well.. it's a fire-breathing flying dragon. And Hydreigon for it's brutal nature and capability to destroy things.