Which level 7 creature is the best, and why?

I think this way: a black dragon can certainly defeat an archangel, but: in the gameplay, the archangel restores +1 to morale and has a high priority, only the phoenix has a higher priority, which means the archangel uses magic first. The most important thing: don't forget about a spell that works on everyone who doesn't have immunity; it works very well: clone. Conclusion: in a game situation, it's better to have an archangel, otherwise, the opposite might happen.