Mainly extreme muscular pain throughout his entire body, even causing him bleeding. It's similar to radiation sickness, and Superman was experiencing continuously for 200 years.
As for supernatural willpower and mind hax resistance, even Superman has layers from that.
Superman is immune to the Anti-Life Equation, which itself has multiversal mind hax and the powers to dominate the minds of everything in the universe, even the higher dimensional god of the Sphere of the Gods. Emperor Joker himself had the Anti-Life Equation yet still couldn't break Superman's will.
Joker even manipulated the collective unconciousness, one of the seven forces in the multiverse. Which is responsible for giving all DC telepaths their power. The Anti-Life Equation is even capable of mind haxxing the likes of the Spectre,
who was powerless against it.
Kyle Rayner was capable of resisting the ALE with his own sheer will.
White Martians were capable of controlling Kyle.
Martian Manhunter could mindhax the White Martians. Superman
can force Martian Manhunter out of his mind and
even do an uno reverse card and reflect the pain back onto Martian Manhunter. He does this again when
MMH tries to enter his mind again.
Superman has also done the uno reverse card trick again when he reflects being mind hax back on the attacker. Superman resists mind hax that
controls every hero on Earth.
All in all Superman himself has multiple layers above mind hax that could dominate the minds of everything in the multiverse. And his sheer willpower can resist things that can affect conceptual Tier 1 structures. If anything, Superman could just reflect the mind hax back on Itachi.
For Itachi using Soul hax to harm him. Superman already has plenty of resistance to that so I doubt if that would even work.
And what's stopping Superman from using phasing to just dodging attacks from the Susanoo, which could phase through Doomsday's attacks, who could interact with Martian Manhunter's own intangibility. So that's two layers above regular intangibility
As for Itachi, melting Freeze Breath, I don't see how that would be possible when it can straight up restrain far more powerful characters. Even then, it would likely take a while before Superman just chucks him into space. Or bring him to the phantom zone before Itachi breaks out. Which he has done for characters he can't contain.
I'm voting Superman if it wasn't obvious