Arceus wins no matter any condition. Giratina is second, Dialga and Palkia are tied for third. Everything else doesn't even matter. Necrozma can't defeat a space diety as he would cease to exist the second he does. Nor does he have some magical time immunity powers. What's stopping Palkia from erasing his location from space and him ceasing to exist? Or Dialga from going back through time, making sure Necrozma never comes into existence in the first place? Arceus exists everywhere and no where, and Giratina is a god-creator of his own separate realm of existence. That aside, lore-wise;
Rayquaza would do very well, Groudon and Kyogre would do well, Reshiram and Zekrom would do well, Yveltal and Xerneas would do okay. Other than that, none of the other mons have a chance. Not even a dick wanked to the thousandth degree mewtwo, or a Regigigas that has been asspulled to be somehow powerful enough to not only catch, but SHOVE THE FUCKING MOON back out into space. Calculate that force for a second. there is no pokemon that physically strong. If he was, he could destroy the planet by punching it.