If any of them wanted to rule Earth and there was no convenient plot armor, which one would realistically remain victorious in the end: an Antediluvian, a group of Archmages, a group of True Fae or the Earthbound? What if instead of ruling Earth their side banded together to destroy the others?

I think either way it is ​Earthbound. Too many sides dislike Vampires much more than the others, the Fey are too susceptible to human Concensus and the other sides, specially Mages, would defect en masse to the Fallen Angels​​ out of wanting power, craving knowledge and wanting a​ b​​etter existence​.

Antediluvians may be plot devices that can escape final death by changing form but any apocalyptic scenario they can cause can be stopped inmediately by Archmag​​es.

Many say Archmages are more powerful than Antediluvians and their true magic (Lilith) is much more versatile than vampire magic (Disciplines, Caine). They are limited by Paradox/Concensus but in an all out war, the Masquerade is broken and magic becomes part of the concensus so there is no Paradox to limit the Archmages.

True Fae only use avatars on Earth and can only be killed in their world, but in​ their​ world they are a plot device where anything they want can be nerfed to oblivion unless you're a fellow Changeling, an Angel, God or a Cosmic Force like the Wyrm.

Because all Demons are Fallen Angels they are beyond broken like having every Discipline at 10 and rigged dice. 'Killing' an Earthbound is almost impossible because of their power and successful killing means forcibly teleporting it back to their realm, to be summoned again when enough worshippers gather. What stops them from taking over Earth is that no mortal is strong enough to be possessed without the body dying from all the power and that they haven't reached the gigantic amount of worshippers needed to summon them in their OP form.