Tapu Koko is incredibly good. Deceptively powerful despite base 95 Special Attack, I wouldn't be surprised if it were the strongest Electric-type in OU. Terrain-boosted Volt Switches and Thunders hit incredibly hard, to the point where Tapu Koko can actually sweep/clean. Its speed tier makes it a bit of a blanket check for the whole tier, and it's really splashable because of this.
Aegislash is superglue. It's one of the best defensive checks in the tier, and will valiantly protect you from half the damn meta. It feels like a bit of a necessary evil at this early point in time, when there are so many things to prepare for. I don't agree that Aegislash is centralizing everything around itself, it feels more like it's holding the tier together, with help from Toxapex and a few others.
Speaking of Toxapex, this little crown-of-thorns starfish that could is a godsend. It checks Pheromosa. It checks Greninja. It checks Genesect. It checks Water-types, some Fairies, etc. Haze is a must, I've already had Mimikyus and Kommo-os try to use it as set-up bait. I'm still working on movesets and EV spreads, it has too many useful moves. You definitely want Scald, Haze, Regenerator, Recover, and either Baneful Bunker/Toxic/Toxic Spikes. Rocky Helmet and Lefties are both good for sustain, and you kinda want to slot a Poison-type move in there somewhere, because Scald does almost nothing to most Fairies. Max physical defense for the EV spread seems like an easy choice, but you could make a sorta-argument for some special defense investment; I've run into a bunch of players that lead with Toxapex and leave it in against Tapu Koko, where it gets taken out on turn 1. Not sure what's happening there, but it keeps happening.
Shiinotic is something that more people need to try. It's got a lot of special bulk, and a huge list of useful resistances. Pair it up with Heatran or Toxapex for a useful defensive core. Access to Spore is always helpful (watch out for Electric Terrain), but Strength Sap is the real eye-opener. It patches up Shiinotic's physcial defense a little, and gives it rocket-powered healing. What the move seems to do is take the target's Attack stat, and transfer that number directly into Shiinotic's HP. You're looking at 65% health restored bare minimum, unless you're fighting a Pyukumuku of something.
Primarina is performing quite well. I'm currently using an Assault Vest set with max Special Attack + HP investment, and it's a surprisingly sturdy special tank. Primarina's Special Attack is only 3 points lower than Keldeo's, so it's got plenty of power. In comparison to Tapu Fini, Primarina has much better offense, while Tapu Fini offers more bulk, Misty Terrain, and Defog for the team. They're both quite balanced; one doesn't feel clearly superior to the other.
Wishiwashi has an incredibly potent bulky Specs set. Pound for pound, it's the strongest Water-type in the tier. Boosted Scalds and Hydro Pumps hit incredibly hard, and it has the bulk to take hits and force switches. Mono-Water STAB is a little lacking, but U-turn helps you keep momentum. Solo form sucks, but it's a slow mon with less than 25% HP. It's dead at that point anyway.
Zygarde is going to be insane once people start using it. Dog form has base 115 Speed, and is a decent revenge killer/check for certain mons. A Coil set with Sitrus Berry uses half the metagame as setup bait, and is unstoppable lategame unless the enemy team has a strong special attacker. Transforming to complete form gives it a huge health/healing boost, and a few Defense boosts make it nigh-untouchable with physical attacks. The lowered Speed makes it kinda vulnerable to fast special attackers, but Extremespeed can mitigate this, provided you have 2 or 3 Attack boosts. Dragon Dance or Salac Berry could also help, maybe Life Orb too. It kinda reminds me of Scrafty. There are a few different ways to run it, depending of how much you want to focus on offense or defense.
Until Thousand Arrows is useable, it's countered by Celesteela. Really wish they'd fix this. Earthquake sucks when Ground and Dragon immunities are everywhere.
Xurkitree is a bloody pain. I haven't tried it yet, but it's given me trouble on multiple occasions. You need an Electric immunity or it's going to rampage. Scouting to see if it's Scarf or Tail Glow gives me a headache, and it feels like it can snowball and sweep way too easily, sometimes without warning. I don't know if it's just me using lots of Water-types or something, but this thing gives me nightmares unless I have Lightingrod Alolan Marowak or Shiinotic.
Alolan Marowak is actually kinda anti-meta. Lightingrod with Ghost/Fire typing lets it check a bunch of stuff, including Pheromosa and Tapu Koko. Thick Club is a must, but I'm not sure if it'd be better to invest in Attack or defenses. Just remember that it's slower than Aegislash.