How is Arceus the "god" pokemon

[tetrahedronX900]

When mega Rayquaza, mega Mewtwo x and y and ultra Necrozma is more powerful than arceus why?

 

[Sweet Serenity]

When mega Rayquaza, mega Mewtwo x and y and ultra Necrozma is more powerful than arceus why?

Notice that all those Pokémon you named are either "megas" or "ultras," meaning in order for them to be as powerful as they are, they need either temporary powerups or fusion with other powerful Pokémon. Arceus is more powerful than their regular forms by itself without needing to fuse with another Pokémon or a temporary powerup such as Mega Evolution. On top of that, the Arceus that we are aware of is heavily implied to not even be its true form. The true form of Arceus has 1,000 arms and the ability to shape the world, meaning that it is likely more powerful than anything that we could ever imagine.

 

[Corveone]

Yeah, Arceus has never revealed its primal/true/complete form unlike the other Pokémon you've mentioned.

In fact, in Legends Arceus lore you can't even actually catch it, you can only get a fragment of its soul (or something like that) that Arceus shares with the player voluntarily. The extent of its full powers are still unknown so it can't be compared.