After a bit of browsing usage rates, I have concluded that Tapu Koko is the most viable in NatDex OU. Used primarily as a lightning-fast pivot (pun intended), sets with Choice Specs Volt Switch are widely used in NatDex OU. The next rankings are (in order), Tapu Fini, Tapu Lele, and Tapu Bulu. However, since they all fill completely different roles (Koko for speedy offensive pivot, Fini for defensive wall, Lele for choice specs powerhouse, and Bulu as a UU physical attacker), it’s all up to preference. Hope I helped!
PS: Tapu Bulu is the only UU of the Tapus, the rest are all OU in NatDex Gen 8.