Which level 7 creature is the best, and why?

well, I don't know... maybe, there are possible scenarios.
I don't really understand the formulas for calculating damage, but due to the damage range of creatures, an Archdevil might actually get defeated... because it has 1.5 times less health, but whether it will deal more than 1.5 times the damage in one round is not a 100% certainty....