I'm just gonna come out and say it that this thing is imo the best pokemon the meta.
Tapu Koko flourishes so hard in this meta, having next to no bad match ups, and acting as the glue to so many good teams. Koko's speed tier is phenomenal and cannot possibly be understated in this meta where speed is so important.
Marshadow, Greninja, Noivern, Mega Salamence, and Mega Pidgeot are all crucially out sped and muscled past.
Sleep Spam is completely negated by Electric Terrain.
It has one of the fastest Taunts in the game to counter Trick Room.
It out speeds and OHKOs all Oricorio forms (yes, even Pom-pom).
Its Scarf sets are a great check to Weather teams, especially Rain but Sun too.
Primarily runs special movesets meaning it isn't crippled by Intimidate.
Eliminates powerful bulky waters such as Milotic and Tapu Fini.
Removes opposing Terrain, particularly shutting down Psychic Terrain strategies.
Provides boosted Electric coverage to it's team.
Acts as an okay spread attacker with Dazzling Gleam.
So it clearly offers a lot, and there aren't many reasons to not want one on your team, but does Tapu Koko really fit into your team? Yes.
There are honestly so few abilities that Tapu Koko doesn't enjoy having, and it has amazing synergy with a multitude of strong pokemon and even play styles. Tapu Koko offers so much by itself that it doesn't even matter if its teammates aren't really abusing its moves and ability, where as Koko gets a lot of mileage out of almost any ability, and uses its solid mixed offences and epic speed to make great use of any coverage or utility move it picks up.
You've probably heard about the Koko Kyurem core. Terrain boosted Fusion Bolt for both. Bolt Beam coverage. Teravolt Taunt, Electric STAB (Lightning Rod, Volt Absorb, Motor Drive), and Ice moves (Thick Fat). On top of just crazy synergy in general.
Rain teams love having Koko to mash bulky Waters and in return Koko benefits from 100% accurate Thunders.
Hail teams greatly appreciate Koko providing some pseudo STAB on Electric moves for Bolt Beam coverage, while Koko itself sweeps up Ice Beam, and sets Aura Veil extremely fast.
Even Revelation Dance teams are actually sick with Tapu Koko, pair it up with Pom-Pom and go to town with Terrain boosted Revelation Dance spam.
Also Alolan Raichu is a thing. I don't think it's the best, but if you build around it a bit and use it well it can be quite devastating. It is also one of few pokemon that provides Speed Swap, which actually has a lot of clever uses in this meta.
When I get time for more games I'll try sharing some sample teams, but until then try building with Koko more, I promise it won't disappoint.
Lastly here's a replay. Yes there's a Koko in it. But I'm more posting this because this team is taken directly and unedited from the Doubles OU sample teams, and I just find it hilarious how crazy powerful it is here.
https://replay.pokemonshowdown.com/gen7partnersincrime-668357528