Which level 7 creature is the best, and why?

Well, I don't really see the point in considering a 1-on-1 scenario, but since you mentioned it: monsters also overwhelm black dragons, but the win rate is lower, meaning it's no longer 100% that the black dragons will lose, but in most cases, monsters still prevail. It all depends on the damage the black dragons deal, and it's unlikely they'll always deal maximum damage.