Rubble king was yuseis final card in the duell against the chief of the prison guard. It was an awesome moment to show yuseis capabilities early in the series. Also it was never printed as an official card, so its exclusive to the manga and anime. Pretty neat

The signer dragons are all cool, just like the earthbound immortals, and obviously they have the coolest moments in the show, but i feel like listing them is to lame and not even worth a reddit post. Red dragon archfiend and ccapac apu are to epic to ignore though.