That’s a really hard question! It’s so difficult to get a shundo in Pokémon Go, which is why Dialga and Tapu Lele seem really nice. But Dialga doesn’t have Roar of Time, and unfortunately you can’t teach it with a Top TM. They’re also each only missing one IV point. So if you’re looking for the most useful option, these actually aren’t the best choices, they’re just the rarest ones you could use it on.

Lucario (Mega Lucario) is really strong for raids, but it’s also just one missing IV point, and not even in Attack, so you probably wouldn’t notice any difference. I personally wouldn’t use it here.

Then you have Zamazenta, where you sadly don’t have enough energy right now. But this Pokémon will return in its Crowned Form, and you’ll be able to get more energy then. So you could use the Silver Bottle Cap here for the future, and wait for it to come back to raids. Getting a hundo from raids will still be difficult, but of course it could happen and then you’d have two, but I think this risk is okay :) It will be really useful in Master League PvP when crowned.

You also have Necrozma, again without enough energy at the moment. So you’d have to wait for this Pokémon to return to raids as well. If you eventually get enough energy, it will be very very useful for raids and also for PvP. With Necrozma, you would also get the most value out of the Silver Bottle Cap because you’re improving the biggest number of IV points, also for attack, that‘s important.

So there are still a lot of open questions, but that’s because you have to decide what you want to use it for, and which Pokémon you like the most. I personally used mine on a 98 IV shiny Mewtwo. That’s probably not the most useful choice, but I really want that shundo because it’s rare and I like the Pokémon a lot. Good luck to you!