Massive poll battle?
The Brethren Moon (or the Tau Voltanis Moon as you encountered it in Dead Space 3), is a massive undead moon formed from the morphed carcasses of various living beings. It is able to devour entire worlds by literally sucking the planet dry with tentacles and shit. When threatened, the moon releases various creatures called Necromorphs that kill people and use their carcasses to make more of their own. The Brethren Moon can also telepathically taunt you or just fuck with your mind and send illusions your way on a massive scale. It will do anything to control you into performing a Convergence event which basically adds more flesh to itself so it can be complete. On the other hand it's really slow and takes awhile to fully devour worlds.
Here's a vid of what several of them are capable of:
https://www.youtube.com/watch?v=NWZlnXz17XI
https://www.youtube.com/watch?v=_1hJQrGwf1s
VS
Sin is a giant (like say, 20 football fields) undead whale composed of the souls of everything its ever killed. In addition to its massive girth and its ability to produce "Sinspawn" from its own body, Sin also has access to the most powerful gravity magic known to man. In fact, its entire being and a city to boot is being held together by the sheer force of gravity. It thus capable of gravity-related feats such as swimming at top speed unhindered, flying, cutting the sea and the world in half (in multiple directions of every vector), and just obliterating entire towns with giant tsunamis. It is extremely resilient and grows stronger with each soul it acquires (in the world of FFX, souls don't ascend unless a ritual is officially performed to do so).
Here's his deeds in action:
https://www.youtube.com/watch?v=lWayMDEiLto
https://www.youtube.com/watch?v=yHzfZ_0dRQo
https://www.youtube.com/watch?v=AsMm524wh9k
There are 3 contests in this battle:
a) Straight up battle. Both beings are in their top form. Who wins?
b) Earth buffet. If the 2 had a contest in killing the most people, who would win?
c) Minion battle. Necromorphs and Sinspawn perform a skirmish on the beach. Which side is more likely to win?
