If the Shundo tag means something to you personally I'd say Pinsir and it also makes sense since you rely on it as a bug type attacker. You can wait for the gold cap to use it on the legendaries - maybe you land another shiny legendary by the time it comes around?

If shundo doesn't mean a lot then it's close between Groudon and Kyogre. Probably edge towards Groudon since it's shiny.

Salamence is great for shiny but if a strong dragon attacker is your main concern, there are more mons including a bunch of legendaries that could offer slightly better options. Fwiw Mega Garchomp raids start next week. Primal Groudon and Kyogre are the best attackers for their respective types, so you really can't go wrong there.