Which level 7 creature is the best, and why?

Well, there was actually proof of this before... the situation is, of course, unrealistic, but nevertheless, the Dungeon creatures were winning against the Archons due to their high Attack and Defense stats.
It would seem that there is no difference: Attack and Defense should balance each other out. But no: in the case of the Dungeon creature, this rule doesn't work because it always reduces the enemy's defense by 80% and, as a result, deals so much damage that it kills even an Archon.

And in a 1v1 fight, with zero stats, there's nothing to argue about. The Archon wins against everyone (and even then, with skillful combat).