J’onn J’onnz - the Martian Manhunter - has been described as more powerful than Superman (even by the Man of Steel himself), and has been a vital part of every team he chooses to be on. As the last green Martian, J’onnz's alien biology allows him to shape shift, giving him the ability to change his mass, shape, color and size at will.
His skills include an enhanced healing factor, plasma beam projection, flight, telekinesis, speed that rivals any speedster, not to mention enhanced strength, and durability. He is one of the strongest telepaths around, using his skills to mentally link every single member of the Justice League of America together during missions.
Manhunter’s telepathy is so strong that he once scanned every living mind on earth at once, fought back against intense psychic attacks, and was able to enter the mind of the Spectre, another immensely powerful figure in the DC universe.
J’onnz's main weakness is a crippling fear of fire. In JLA #34 J’onnz is able to overcome that fear and transform into the villainous Fernus. As Fernus, he was able to battle the Justice League single-handedly, before being stopped by Plastic Man.