In-game, probably the Battle Tree. The Battle Royal (in my opinion) is very annoying, and the CPUs seem to just yeet all their attacks at the player, plus the scoring system is weird, while the Battle tree is straightforward 3-on-3 battles, where the formula is: You keep battling til' you lose, or decide to save your streak and take a break. And the Battle Royal awards a set amount of points each time, while the Battle Tree's reward increases the larger your streak gets, so if you have strong Pokémon, or are good at battling, do the Battle Tree. If you are playing normal Sun, the only 2 options are Battle Royal and Battle Tree. (If you thought the Battle Tree was hard, try the Battle Royal...) however if you have Ultra Sun, Martine Surfing might be the best way if you are struggling with battles.
