In the original 1st edition Monster Manual, Demogorgon was easily the most powerful demon lord, while Asmodeus was the most powerful arch devil. Between the two, Demogorgon was probably a hair more powerful than Asmodeus.
Asmodeus was technically a deity, but was exempt from many of the rules regarding divine beings, his actual status a matter of debate that had changed over time. At the very least the strongest archdevil, weaker interpretations cast him as not even a full-fledged god or possessed only of the powers of a lesser god.
