Tapus, hands down.
Both the Ultra Baasts and the Tapus are meant to be competitively used pokemon, and I will fully admit to this. But the Tapus fill a role that's in pretty much every generation. They're this region's "Trio" ala the birds, the beasts, the Regis, etc. They fit into Alola incredibly well, and removing them would feel like removing a large aspect of Alola itself.
With Ultra Beasts, it's very.. different. Back when they were first revealed and we didn't know if we could catch them or not, we just thought they were going to be boss pokemon ala the Totems, except they would be made specifically for the bosses. So, yeah, they would be minmaxed anyways, they didn't have to limit themselves in the same way as they did here. They wouldn't have had to restrain themselves to a 570 BST because they wouldn't be catchable, and we would have boss fights with incredibly strong pokemon.
But since they were catchable, Gamefreak had to balance them out with the rest of them, so in the end, they feel a lot more like the Genies of Gen 5. Pokemon that don't really fit in with the region of the game and are just there for competitive usage. Yes, the Tapus are the same, but unlike the Ultra Beasts and the Genies, they fit in with the region and lore they were introduced in. The Ultra Beasts could've been introduced in any other generation and fit in pretty much as well, if they had used pretty much the same story. The only real connection the UBs have is that the boxart pokemon are UBs and they kicked the Tapus' asses centuries ago.
