Zarathos vs Mephisto

Who would in this battle

I remember when GR made his way to hell, I don't remember the comic though, a couple of the demons cowering behind Mephisto asked "Zarathos?" and another yelled out "The rebel!"... so I guess Zarathos is just a rebel demon.

Zarathos would win only if he was equiped with the xenon upgrade.

yeah but in his caporial form zarathos is a straight up badass, he would probably and has been a very formidible opponent for maphisto but still wouldn't win.