Read "Trial By Fire," pre New 52 JLA story arc. I won't spoil it, but at one point, Superman says: "I can count on one hand the number of beings in the known universe I would be afraid to face in combat. J'onn J'onzz is at the top of that list. He is the most powerful being on the face of the Earth." In the arc, Martian Manhunter basically, effortlessly takes out the entire JLA single-handedly. And not in a corny way, it is very believable (as the story arc also effectively removes his one rather crippling weakness to fire). It's a neat story, worth a read if you like MM.