Volcarano and Excadrill.
Now I must say, Excadrill I had on my team. It swept everything and never died. However, Clay's Excadrill, completely swept my team, besides my Servine.
Volcarano, almost knocked my team out untill Excadrill got a critical Drill Run. (Didn't teach my Excadrill Rock Slide, didn't like it on him)
Though, Ghetsis Hydreigon, was pretty tough, I EV Trained my Haxorus in speed, and it didn't OHKO when it hit.
I would say, Hydreigon wasn't as difficult as people make out.
I say the most overpowered were, Volcarano and Excadrill.
~Totodilesteel99