A Message From the Actual CEO of GSC NU
Some general thoughts. Apart from S rank, order isn't really all that big of a deal.
S rank
Xatu - Still the best in the tier. I don't feel the need to have to elaborate on this one. Hp electric is almost always better then ice, just in case you run into a rest talk war against Octillery. Although it does make it worst if you have to use rest talk on a dugtrio. nvm idk
Octillery - I am high on this mon currently. So much so, I think its 2nd best right now. but I am also going to argue that it has qualities to place it alongside Xatu in the S rank. I think Xatu is the glue that holds the tier together, while Octillery is the glue that holds your team together. You place it on one team, and you have a secondary xatu pressure, a dugtrio/sudowoodo/graveler counter, you have your Ninetales/any other fire type swap, you get offensive presence with strong surfs, can even somehow function as a general sponge (in some cases at least). It pairs very nicely with other Pokemon as well like Ninetales, Gloom. Usually have to answer this mon with your own Octillery/water, or Gloom, but even then gloom can get sniped on the odd ice beam set and it sucks to go your own Octillery on a Hp electric. It's just absurdly easy to throw onto just about any team with no consequence and only needs one set to function despite there being some other options and variability. I used it as a defensive glue in teams while not sacrificing any offensive output thanks to its strong STABS and great SPA. Excellent mon with basically no drawbacks to use.
A+ rank
these are ordered, but all are close enough that its not worth arguing on over the other.
Weezing - Its still as good as ever. I don't think this placement is of much dispute. I like the 3 attacks set, but haze is also really nice. Pain-split is an interesting option as well. Prefer to avoid Hp grass due to loss of DV's.
Primeape - the usage stats showed that by week 4 of GSC PL, Primeape was used 3 times total. I think two of the three was Lax. Earthworm's Thunder set is really good, should 2hko a Xatu + added benefit of paralysis. Can mix it up with meditate and rockslide as well. Personally, I think its a undervalued mon based on usage. Its fast, decently bulky, great STAB, great move-pool. Top tier threat in my opinion and its rather easy to slot into most teams with little to no consequences.
Kingler - Kingler and Primeape are your premier offensive options in the tier. base 130 attack is no joke, and surf or hidden power gives it rather good coverage as well. its not that slow as well, so after a SD, other bulky waters can really deal with kingler well. it seemed that people relied on a Weezing or Xatu to stop it, but doing so meant that traded a large portion of of HP.
Stantler - I don't echo the exact sentiments of estarossa above with it as a S- ranking, but it has seemed to be more understood with higher usage as well. My gut feeling is its high usage has been because of the rather low usage of Primeape and fighting types. But that said its rather fast and bulky, and most annoyingly can have variation to the sets that are all dealt with differently. The combinations of curse rest talk, eq, light screen, thief, hypnosis, normal STAB make it hard to deal with in the builder. The most common set of Curse 2 attacks Light screen (or similar variation) is rather annoying to face as well. I personally think a lot of its strength came from this variability in its sets, as each one has a different approach to stopping.
A Rank
W
igglytuff - bulky normal that has some variability in its set options. Very easy to fit on fatter builds and curse rest talk is always going to be a strong threat to opponents. I have always been pretty high on this mon, and I have quite a few fat teams with it.
Ninetales - I like Ninetales a lot. I don't know how much I need to elaborate on this mon, most everyone knows its good. Octillery + Xatu + Ninetales is a common core I run on teams that can cover most of the threats in the metagame. the Reflect Flamethrower rest talk set is a really interesting choice, while toxic is almost equally as good. Its a very good defensive choice that can still threaten non water types with a decent STAB flamethrower. Strength is in ease of fitting into teams, bulk, and speed.
Dewgong - If I am going to use a bulky water as a defensive sponge, Octillery is pretty much better I think. I might be just to high on Octillery at the moment though. Really only used this mon to toy with the encore, toxic, ice beam, protect set as a method to deal with the 2 attack light screen curse Stantler sets. Admittedly, STAB Ice and Water is really good along with better speed than Octillery. I think the biggest drawback to Dewgong is also its Ice typing. Players count on their water slot to deal with a Dugtrio and/or other ground/rocks. Essentially, the better bulk doesn't help you out when it counts. Being ice type makes it to suspectable to rock slide hits and flinches. Although you want to answer Fighting types with Xatu or a Weezing, sometime you gotta use your bulky water to do so. Dewgong being weak to fighting STAB sucks when it should be adding defensive bulk to the team. Still good overall, but harder to fit then Octillery.
Gloom - Also used gloom a lot, but never felt the need to get creative with its move pool. It fits really well alongside Octillery, creating great defensive backbone on teams. I preferred the HP ground as a fourth option so the odd Magnemite can't just sit on it, but there is a ton of variability, such as sleep powder or even Hp ice so Xatu isn't free into it. Hp grass is cool too so Kingler can't use Substitute on it. Its also nice secondary fighting check, but unfortunately not really the pseudo normal check that Weezing offers builders. really good when paired with waters, massive downside is Xatu is pretty much always a free switch on it.
Dugtrio - Always a threat and always a potential pain. Screech can be really annoying to deal with. Its fragility and lack of versatility keeps it from being a A+ rank mon personally. It's STAB Eq is strong, but it isn't
that strong honestly. I don't think you can throw a Dugtrio in teams as easily. Any good team has good answers to the Dugtrio as well. Regardless though, Dugtrio will always be a threat in the tier.
A- Rank
Chinchou - I think Octillery is as good as ever, so Chinchou would be as well. Its less splashable then other waters though, but has the benefit of being a free swap into said opposing water types. I personally lean towards always using the 3 attacks set, because Chinchou's bulk sucks and I pair it with octillery more often then not, and would prefer to not have two mons walled by Gloom.
Pineco - You realistically get spikes and a boom out of it. but spikes makes this mon relevant. Reflect sets are nice, and so is toxic. the biggest drawback is not feeling like your wasting a slot in the teambuilder. Pineco really isn't going to do much else.
Sudowoodo - my perception of this year was more normal types then usual. It fits well into most teams and gives builds an explosion option as well. Downside is Octillery abuses it. So yes this mon is good atm even if Stantler learns Eq.
Magnemite - Strong stab thunder + can come in on Xatu ok-ishly, really good mon. It could be a B ranked mon because its not particularly bulky and relies on its steel typing to survive hits. its also slow, but I think it makes up for it by excellent coverage and power.
B+ Ranking
Magmar - Speed, decent bulk and excellent coverage. considered A rank.
Hitmonlee - Under utilized kinda, has great attack and decent SpDef. Admittedly though, it is hard to justify over Primeape.
Porygon - Hard to kill general sponge, Recover+ Boltbeam + t-wave is all it needs to thrive.
Persain - Speed and Hypnosis. Its not bad, but I also prefer bulkier curse options.
B and below Ranking
Fearow - I actually don't like this mon, but its has respectable offensive presence.
Flareon - Baton pass is also underused atm. Flareon can have potential for other uses outside of Baton pass as well
Rapidash - I haven't figured this mon out, I prefer Magmar as an offensive fire, and Ninetales as a defensive fire usually.
Graveler - Worse Sudowoodo, but nice if you need to spin. STAB Eq is cool though.
Azumarill - I think its good for the lightscreen support and excellent bulk, but its passive and can be set up fodder as well
Houndour - Situationally excellent. Has some interesting synergy with things like Gloom, Kingler, and Weezing as well.
Shuckle - It's probably much better then I'm aware of. Not sure how to rank tbh.
Dragonair - Use to be a common lead, seems to have fallen out of favor though. Not sure how to rank tbh.
"The Rest" Ranking
I don't think there is a need to sort the rest into rankings. They can all appear viable in certain builds, and there is plenty of unexplored potential. I admit that most I don't know well enough to even properly rank them. Not a comprehensive listing.
This is just my 2 cents; feel free to @me with what you think is a blatantly bad take.