Vote: Xen's Excadrill.
Because this is such a difficult vote, I'm going to go with each mon one by one and explain why I'm not voting for them, taking into account both 1v1 situations against koko and general performance in the current metagame. Because everything this week could actually counter Koko in one way or another, I'm going to give my opinions on why each mon is good for the job, but also how it can miss out in other aspects.
Alowak - Parivard: Amazing counter, switches in on almost every attack but has to worry about HP ground which is a clean 2HKO. This puts the player in a 50/50 situation as to weather you stay in predicting the hp ground and retaliate, or still switch to alowak as a general counter. 50/50 scenarios are most definitely one of my least favourite things to encounter, which is why Alowak isn't my choice (partly along with how Alowak performs in general outside of the pre-bank metagame).
Leader Wallace - Lanturn: One of my favourite counters here, purely because of volt absorb. It tanks any hit, including grass knot / hp ground really easily, and can chip back in return. In a 1v1 situation Lanturn is going to win pretty much 100% of the time, but outside of this, Lanturn kind of sucks as a bulky water, being weak to ground which is not what you want since majority of bulky waters are going to be your ground-type answer. So, Lanturn almost got my vote but not quite.
ThickFatAzumarill - Mega-Venusaur: An extraordinarily clean counter to Koko, only losing vs the occasional subpar set of supersonic skystrike. While skystrike isn't the most popular set around, it still exists and can prove to be rather troublesome for Mega-Venusaur. In addition to this, while Mege-Venusaur does provide a fair amount of viability in the current metagame, it competes far too heavily with other mega pokemon for that mega slot. While doubling up on megas does remain an option, Mega-Venusaur isn't the kind of mega I would want to have as a double mega. Extremely close, but Venu misses out.
Xen - Excadrill: Excadrill takes my vote this weak. Literally only has to worry about HP ground, and even then (given that Excadrill is scarfed) you survive the hit and can OHKO with earthquake. Scarf also provides great speed control and beats Mimikyu 1v1, an extremely threatening Pokemon right now. Also pressures bulky mons with Horn Drill such as Porygon2 and Celesteela. Because of these in tandem to countering Koko, Excadrill gains my vote!
Kanon90 - Magnezone: A clean counter assuming that Koko isn't running HP ground. However, assuming HP ground (which we have to do given that a counter is meant to switch in and beat the threat regardless of the situation), Magnezone won't be a counter. It's a clean 2HKO and can't outspeed, so bringing it in as a counter is extremely risky. In addition to this, you only have a measly 6.3% chance to OHKO. Even in a 1v1 situation, Koko has a huge advantage. Without HP ground I absolutely love Magnezone as a counter, but that possibility alone is too much for me to vote for Magnezone.
Nelson Tangela - Thundurus-Therian: Does literally the exact same thing that Excadrill does, but rather taking the hit from HP ice and providing a OHKO with sludge wave. The only reason that Thundurus-Therian doesn't gain my vote is that Excadrill has a much better matchup against those top tier threats, which Thundurus-Therian can struggle with. With this it wasn't a matter of what counters Tapu Koko better (since neither does it better than the other), but rather which one more appreciates the metagame it is in. To me, Thundurus-Therian isn't the answer.
3heartstchall - Nihilego: Again, pretty much the same argument I made about Thundurus-Therian. I simply believe that Excadrill, while countering Koko all the same, also threatens top-tier Pokemon a whole lot more more. The exact same case applies to Nihilego in this instance. Close, but not quite!
TheSquash's Rhyperior: A rather interesting counter. The biggest problem for me is that it still loses to grass knot, which has an 18.8% chance to OHKO. Far too shaky for my liking, while also rather slow and underwhelming in the metagame itself. Not entirely sure if I want this to be my Assault Vest user either, with Rhyperior still packing a whole lot of weaknesses which Assault Vest can't cover up well enough. I must admit though, very creative and I do like that.
Mike81401's Mega-Steelix: Literally counters Tapu Koko better than anything else here. Grass knot / HP ice are nothing but a tickle, being a guaranteed 4HKO (this is actually pretty insane lol). My problem here is the same as with Mega-Venusaur to an extent, being that it has far too much competition as a mega. Plus, Mega-Steelix has a really really horrendous matchup against offensive grounds. Looks like a budget Hippowdon to me. Though I do admire the balls to submit this. Great submission but not quite enough for me!
So, yeah. Fantastic submission though everyone, really happy with this week! :]
