One hot topic of the moment because let's face it, Mega Rayquaza got competition.
If you really ask me, it's just a matter of luck. They are both crazily powerful, they both got insane abilities to cover for them.
Sure Mega Rayquaza got better attack and special attack stats, but Ultra Necrozma is faster and neuroforce is to be reckoned it. Anyone who already beat the game can relate that, that oversized light bulb is not laughing matter.

So yeah it's just like scissors-rock-paper. You can't tell who will get wrecked until get to see each fight.

Ah please don't make this like a forum discussion post, this is just a silly illustration made for fun just enjoy it. Regardless you favor one over the other please have in mind that's mostly personal opinion and there's absolutely nothing wrong with that. But.....be civilized that's all :3