The Tapus, Or Necrozma
I actually suggest to use at Tapu Fini, since many people struggling to catch Tapu Fini because of its aqua ring which makes it heal itself everyturn. Tapu Fini also has Muddy Water which can reduce your Pokemon accuracy, and making your Pokemon harder to hit it. Necrozma isn't really that hard, I use my masterball to catch Necrozma because I ran out of pokeballs. Its move aren't really that bad, but it has Iron Defense which can be annoying sometimes. The other tapus isnt that bad aswell. So I suggest using the masterball on Tapu Fini