I prefer Mew, even though Arceus is superior. Mew with cute. Arceus had a fugly voice in the dub.

If we go by anime, Mew can transform into any Pokemon at will, and probably into any type of Arceus, so Mew can win there.

"And The only Mew shown to transform at will is the ones that bonded to that Tree."

Well, Ditto is obviously inferior, as it has to see a Pokemon or object to Transform into it. I doubt Mew would need to, seeing as it knows every Pokemon in existence.

"Arceus can simply Change to a fighting time via Fist Plate and Slaughter Mew with FIghting STAB Judgement... "

And Mew could easily change to another Pokemon.

If we go by game, Dark Type Arceus vs. Mew, I doubt Mew would survive, unless it had Choice Scarf, Transforms, then uses some move defeat Arceus (which probably has Focus Blast or something to if the user knows Mew has Transform).

"Mew does not so if he transfomrs to Normal Arceus he still lower Stats because Transform doesn't copy stats ."

Transform copies every stat except for HP. The max amount of HP Arceus can have is 444, while for Mew it's 404. Not too much of a difference.