I’d lean towards MM. Supes has admitted J’onn being one of the most powerful cats on earth and not someone he want to fight in open combat. One of the best things about MM during new 52 was the aggressive way he used his abilities in a fight. On paper J’onn should dominate depending on how his fire weakness is treated.
I’d say Martian manhunter, his physical powers are pretty impressive, even for Superman, but the real kicker is his psychic abilities, which Superman doesn’t really have a counter to.
Obviously Martian Manhunter. He can take on characteristics of anyone including aliens. He would have at least the equivalent powers of Superman.
Superman. He’s faced worse. Martian Manhunter will be paralyzed with fear after one flame appears. And let’s be honest, he is overrated in power, usually ends up getting taken out easily.
Superman because of the heat vision.
Every power they share Superman is more powerful with.
Freeze breath can stop him from shapeshifting or phasing.
Heat vision not only has the power to melt him, but mentally incapacitate him.
Zod and new 52 superboy beat the hell out of jonn.
Only way he wins is by Superman jobbing, which he does often, so he might lol.
Btw if we are also using tv versions, then the Supergirl tv shows MM cant use his telepathy against kryptonians.