Which level 7 creature is the best, and why?

I personally just used autobattle to test them. With equal stacks and hero stats of 1/1/1/1, earth and sand elements, no morale or luck, and neutral specializations without hero abilities, Dr. Behemoths defeat Archons in about 3/4 of the battles. With morale, it's 2/3. And this applies to any number of units. My vote goes to Behemoths.