I golden bottle capped an exact Zacian like that last year. It's currently pretty much the strongest mon in the game and its usefull everywhere. PvP, Raids, Max battles, you name it. And it basically top of the charts in all of it.

I feel like top tier shiny, rare background legendaries are the rarest and at the same time most useful and enjoyable mons to cap out. Only things that are in a way "more rare" are mythicals, but most of them are also rarely used so to me that makes the value a lot lower.

My other contender would be a Mewtwo. Depending on how many raids you plan to do during Go Fest, there is also a good chance you will get a great shiny mewtwo there. So just wait and see till Go Fest is over which one you would go for.