Probably Osiris/Slyfer due to his ability and how OP the Slyfer/Slime strategy was (at least in the anime/manga), plus it doesn't require sacrifices to buff it like Ra, though Mega Ultra Chicken has the kewlest design.
Winged Dragon of Ra for being a mechanical golden dragon and also being a goddamned beast in the anime. Though I'd be more likely to use Obelisk than Ra in a competitive field, as Obelisk has the best effects out of them in the TCG.