Which level 7 creature is the best, and why?

IMHO, Behemoths, especially Ancient ones, are good in battles against strong opponents—level 5-7 units with high defense. The higher their defense skill, the more advantageous it is to attack them with a Behemoth.

For example, considering the fight with an Archangel I described above, the Behemoth has a significant damage bonus because the Archangel's high defense is reduced from 30 to 6, and thus, the Behemoths' attack increases significantly. On the other hand, when attacking, for instance, Centaurs with a pitiful Defense of 3, Behemoths receive no additional bonuses.

Added after 2 minutes

he always seemed a bit weak to me compared to the other 7s)

That is exactly how it is: compared to other level 7 units, the Behemoth is far from the best fighter. And its magic immunity often feels irrelevant: for example, you cannot help it with Shield or Stone Skin. :)