I should preface my opinion in that I think that three of the four tapus are very close to each other in viability, which is why I thought it might be interesting to rank them.
Fini:
For me, the reason I've put Tapu Fini as the best is I think it's role is so pivotal in the meta. I've always been of the opinion that bulky offence, though only by a little bit, is probably the best style in BSS. Tapu Fini for me is a key part of bulky offence, generally combining everything you want from a great defensive backbone pokemon. It's typing is fantastic, allowing it to switch in and take a hit from Mega Salamence, Mega Blaziken, Charizard X, Mega Metagross, Mega Gyarados, Heatran, Celesteela and Mega Mawile, to name some top offensive threats. It is no slouch vs. defensive mons either, with access to taunt and natures madness, allowing it to potentially 1v1 pokemon like porygon2, aegislash and celesteela. It is very difficult to find a pokemon that can ohko fini without a boost. Even Specs Tapu Koko's thunderbolt in electric terrain, which you would think easily KOs only has a 6.3% chance of KOing max HP tapu fini, which fini can Ohko back with waterium. After Mimikyu, Greninja and Landorus-T, I think the next most splashable mon is Fini, you can't go wrong with it if you need a bulky water, it will put in work every single game because it can scrap with pretty much everything at least chipping off health.
Lele:
Lele for me is the second most important tapu in terms of teambuilding, but in a different way than Fini. Fini is like an answer to a wide variety of threats, almost filling a similar role to a rk mimikyu. Lele is like a mega salamence, something that you always have to keep in mind when building a team. Ever wonder why there is a steel type on practically every team? Yes, there is mega salamence, but pretty much its just for this one mon. Anything that isnt p2, snorlax or a steel type will pretty much drop to specs psychic and yes you can have dark types but they hate moonblast. Not much more to say about lele other than its psychic hits with the force of a freight train and its terrain is very useful for certain types of setup or psychic spam teams (with mega zam).
Koko:
Koko is very versatile, a fast taunter with momentum, shuts down breloom in particular and is decent versus quite a few greninja sets. Electrium-Z is a key set because without it or a choice specs, it can be a bit weak compared to a lot of pokemon on the battlespot, which is where I think its slightly worse than the other two. I do think that Koko is probably the most versatile of the three though, its support and setup sets in particular are all viable and very annoying to face.
Bulu:
Sorry bulu you are 4th. Not much to say but that you get ohkoed by all five of the best megas in the format and you are slow :(
