In terms of ATK and DEF points F.G.D is the best (And Dragon Knight Master), however in reality no such as strongest exist, it is up to how you play them, though Chaos Emperor Dragon - Envoj of the End (super easy, almost total lockdown) or Pot of Greed (almost instant +1 at no cost, must for any deck if you play in traditional format), but again there is no such as strongest!!!