Following the most recent tiering update to GSC NU and in anticipation of an upcoming RoA rotational ladder featuring GSC NU, now seems like a good time to post my analysis on the current metagame and what I think is good. I admittedly haven't been able to play many games yet but I've been staring at a list of the Pokemon for a while and this is what I've been able to come up with.
These three seem like the main offensive powerhouses in the tier. All three are special attackers that were held back by Raichu, Poliwrath, and Golduck in the previous iteration of the tier.
Octillery, which was a top Pokemon in the Mr. Mime Pikachu meta, is finally back to wreak havoc with its base 105 SpA. Almost nothing in the tier can wall three attack Octillery, but unlike Golduck and Poliwrath, it's pretty slow and not that bulky. It has to choose which coverage to drop if it is considering going RestTalk. HP Electric does substantially less than Ice Beam to Xatu and obviously very little to Grass-types.
Xatu's Psychic is scarcely resisted and comes off a respectable base 95 SpA. This coupled with its high Speed and Spikes immunity means that Xatu can safely be considered a powerhouse of the tier. However, unlike Octillery, it struggles to cover all of its checks with its available coverage moves. Xatu can use Hidden Power Dark or Electric for opposing Xatu and Mantine (and to a lesser extent, Exeggcute and Abra) but fails to hit Pokemon like Ninetales and Magnemite hard. Nonetheless, these Pokemon are threatened by critical hits and SpD falls from Xatu, and Xatu has other tricks up its sleeve like Night Shade and Thief that can offer means of bypassing some of its trickier checks. Xatu is also arguably one of the best Pokemon at taking advantage of chip damage from Spikes thanks to its immediate solid damage output and high Speed.
Flareon's high stats make it a significant standalone threat, with its STAB Fire Blast being backed up by HP Grass for Rock-types and Zap Cannon being a stronger option for Water-types. Its huge Attack stat can also be taken advantage of with Double-Edge or Hidden Power Ground to deal sizable chunks of damage to opposing Fire-types. However, the main draw to Flareon is its access to Growth, which is made even more dangerous when used with Baton Pass. Xatu, Magmar, and Seadra were already looking scary with no boosts, but Flareon can take them to another level. Not only this, but Pokemon like Ninetales and opposing Flareon with Roar can also be foiled by slower Flareon variants with Roar. Flareon has plenty of options to make itself incredibly dangerous and should not be underestimated, even though it can struggle to find safe opportunities to set up due to its typing and susceptibility to status.
Without Raichu around to deal massive damage with Thunder, one might think Normal-types would demolish the metagame with high base power STAB attacks and setup/coverage moves, but there are still annoying hurdles for them to get over before they can hope to sweep. Shuckle, Graveler, and Sudowoodo are relatively unchanged. Weezing is still around, as is Gastly. Magnemite is now also seeing significantly more play, and Corsola's viability has substantially increased. It can be hard to consistently find a way to break the opponent's defenses against Normal-types, although those with Earthquake are in a better position than those that don't have it.
Pineco arguably remains the more viable spiker, however Delibird is more viable than it was when Raichu was the most popular Pokemon.
Weezing is still a very good Pokemon and can really cause havoc if it has an attack to bypass the opposing team's Normal-type resist.
Graveler is notable for Rapid Spin, STAB EQ, and Explosion. Sudowoodo has much better SpD and no 4x weaknesses, so it can't get destroyed by random Hidden Powers. Shuckle is a super tank but is incredibly passive and arguably struggles in a tier with fearfully powerful Octillery. Corsola's niche as a non-Water weak Recovering Rock-type is now more viable due to Raichu being gone, but Grass-type coverage is a concern. Magcargo is a solid check to most Fire-types if they lack Cross Chop, Hidden Power Ground, and Hidden Power Water. It can permanently wall Curse RestTalk Normal-types with Acid Armor.
While they can't take on powerful Normal-types reliably, bulky Water-types are crucial for taking on Fire-types, Rock-types, and Ground-types. Octillery has a solid balance of power and bulk, whereas Dewgong has STAB Ice Beam for Xatu and more bulk but less power. Mantine is fantastic in terms of SpD but has an unfortunate 4x weakness to Electric. Nevertheless, it is potentially valuable for its consistency against non-Electric move Xatu and Flareon. Azumarill is the most bulky of the four, rivalling the bulk of Poliwrath. However, it has a paltry base 50 SpA, so it is quite passive outside of Whirlpool + Perish Song and Belly Drum.
Fire-types are more potent than ever, although they are somewhat lacking in ways to get past bulky Water-types due to lacking Solar Beam. Flareon has Zap Cannon and Magmar has Cross Chop for Dewgong as well as Thunder Punch for Mantine. Rapidash relies on Double-Edge or Hidden Power alongside Hypnosis. Ninetales provides a great option for checking opposing Fire-types and Xatu thanks to its high SpD. Magmar is particularly potent with Sunny Day boosting its power to extremes. It nevertheless struggles against some of its checks, like Octillery and Ninetales.
Other Physical Attackers
Physical attackers aside from exceptions like Lickitung are for the most part overshadowed by special attackers, which boast better defensive attributes and coverage. However, given a setup opportunity, physical attackers can be highly effective at dismantling a team. Kingler is highly threatening given an opportunity to set up Swords Dance. It is a great fit for a paralysis-based team. Primeape is a fearsome attacker that Speed ties Xatu and has Rock Slide. Dugtrio is excellent with Spikes, although can struggle to find as many opportunities in the new metagame. Hitmonlee has great Attack and SpD, but lacks coverage options. However, it can also function as a spinner. Poliwhirl is uniquely threatening with its high Speed and access to the combination of Lovely Kiss and Belly Drum. Machoke has access to Cross Chop, Earthquake, and Rock Slide. Cubone has the highest Attack in the tier with Thick Club alongside STAB Earthquake. However, both Machoke and Cubone are very slow.
Electric is still a useful and powerful type, despite only NFEs being around to use. Magnemite has extremely useful typing and a highly respectable base SpA of 95. In the absence of Raichu, it is a clear mainstay in the tier, being an excellent check to Xatu and Normal-types without Ground type coverage. Elekid is fast and has useful coverage, making it surprisingly difficult to counter. Chinchou has a valuable Water-resist and may be the only Pokemon in the tier capable of taking on three attack Octillery safely. Voltorb is primarily useful for its access to dual screens and Explosion.
Other Special Attackers
Seadra has Agility so it can outspeed the entire tier and threaten it with its great coverage. This is even scarier with potential Growth boosts from Flareon. Abra doesn't have the coverage issues Xatu has and has incredibly high SpA but is really pathetic defensively.
Other Pokemon that might be viable
These are some Baton Passers, Grass-types, Dark-types, and Pokemon with unique niches that might make them viable. I might have missed some Pokemon.
I hope this is helpful to anyone that is thinking about trying out the new tier in 2020. I left this slightly vague and I think there are many possible strategies to experiment with here so I am excited to see other people's takes on the tier. Please post about your experiences with the tier in this thread. What sets and combinations are working for you? Is the tier more enjoyable after the bans?
See you on the ladder!