Good question. I always felt the original incarnation was the most powerful being in existence but limited by the fact that his master, Johnny Thunder, was kind of a dim bulb. However, DC Fandom has this to say: He cannot be ordered to kill, he must interpret commands literally so you have to be careful how you speak, and as an electrical being he is vulnerable to water.
They also add that his summoning word, "Cei-U" pronounced "Say You" (or "So Cool" is close enough) is actually what his name Yz sounds like backwards. So can he be banished by saying his name forwards, sort an inverse Mxyzptlk?
I always saw Maxy as most powerful Imp. or maybe most important one, due to him being foil to Supes, but that Imp in pen would be most powerful on that earth!
Think that you are right about precise wording, as have to command him to do a specific act, not just say stop Injustice league!