Now that I'm not just typing a rather halfassed post---
I'm going in with the assumption that we will ban Tera, by the way, but I'm going to mention stuff here when I think it's worth it. I'll also not be mentioning anything from yesterday.
===============LOSERS (mostly)===============
Rain lost a
lot of its tools this time around - while the Hurricane trio of Dragonite, Gyarados, and Salamence remain, Tapu Koko is gone, every viable swift swimmer bit the dust, and you really don't have any strong and decently fast Thunderspam user left. Drednaw is the only 'good' Swift Swimmer, but itself has a poor defensive type and only has Hydro Pump, Surf, Ice Beam, and Earth Power to its name. Shift Gear Toxtricity will likely remain as useful as it always has been, but now serves as the 'fast' Thunder user. On the upside, Rain gained Iron Bundle, who has extremely good raw Speed, high Special Attack, Hydro Pump, and Freeze Dry... in standard. Here, it relies on Ice Spinner and Liquidation, and has a far stronger Flip Turn than before. It hits like a fucking meteor, but it doesn't have a boosted Speed stat to avoid getting jumped by a random Scarfer and misses out on Hydro Pump.
Rain gets significantly better once everything that is unreleased is properly released - Thundurus-I and Zapdos can serve as extra Thunder users, Tornadus shows up for Hurricane, and perhaps most importantly is Basculegion, who has a much better typing and coverage (including both STABs!) give it a way better place than Drednaw as a Swift Swimmer. And Last Respects will hopefully be banned.
Tera can help patch up several holes, such as allowing Roaring Moon, Slither Wing, or (eventually) Lilligant-Hisui to join the fray as Hurricanespam, but in general Rain just lacks the same speed-and-power versatility it had previously thanks to Koko's omnipresence, but it's still gonna be pretty damn strong.
Ghost Special attackers suffer the greatest nerf of them all: Poltergeist no longer exists. Polteageist and Rotom barely existed, particularly Rotom, but Gengar lost the most important took in its entire gameplan, and its best Ghost STAB is now Shadow Claw. Tera Blast doesn't patch this issue, although at least Polteageist can act like it's cool by changing into anything but a Ghost type.
Magnezone lost a lot of its favorite switchin opportunities in the Tapus, many other Fairies, and there's just far fewer Steels for it to abuse, especially Ferrothorn. Iron Defense + Body Press is what defined it and its ability to handle Ferro easily, and that is now just completely gone. Corviknight is the only reasonable target, as everything else is either uncommon or outspeeds and OHKOs it, such as Lucario or Iron Treads. Speaking of which.
===============WINNERS===============
With the massive loss of an incredible number of common defoggers and a lack of Steel types with good movepools and good stats, Iron Treads breaks the mold by being not just good, but
incredible. Treads rocks one of the best statlines, a great typing actual dual STAB options, a pivoting move in Volt Switch, Rapid Spin, and Stealth Rock. Iron Treads exists as an Excadrill replacement who trades Sand Rush for Volt Switch and better overall stats, particularly Speed. Treads functions in Electric Terrain teams for Quark Drive, Rain teams as a mass resist/spinner/Thunder user, or just in any team in need of hazard removal.
It also gets Steel Beam, which if you know me at all, is the best gift of all.
[imagine the rest of the Protosynthesis mons and Ceruledge here lol]
Sun benefits from Rain being a bit less prevalent. Oh and the fact that a bunch of the new mons are designed exclusively around Sun.
Of the Protosynthesis mons, Slither Wing and Roaring Moon stand out; Slither Wing has U-Turn, Bug Buzz, Heat Wave, healing in Giga Drain, and more reliable healing in Roost as well as Bulk Up to snowball if you so desire, and gains a massive power increase under Sun. More impressively is Roaring Moon, who much like his friend Salamence comes with the benefits of one of the most expansive movepools in the game, including U-Turn on top of Salamence's normal fare of Fire Blast, dual STABs and Dragon Dance.
Lots of other Fire types also are helped out. Ceruledge and Amarouge both have extremely strong STAB moves that they love to spam, and granting them additional power as well as reducing Water power is excellent. Scorvillain is the coveted Fire/Grass Pokemon we've been waiting 4 generations for, and while its raw speed and power are underwhelming, its access to Solar Beam, Fire Blast, and Stomping Tantrum is enough,
especially with Growth.
Slight note on the Chlorophyll sweepers this time: Leafeon lost Weather Ball in the generational shift, meaning it now needs to rely on Stored Power or Mud Shot (ew) to break past Steels. Lilligant-Hisui, when released, lacks a Fighting move, and while Tera is legal, that is likely the best option for it.
[imagine this is clodsire]
Clodsire is cool. While losing the Tapus makes having the Poison typing a bit less cool, Clodsire has enough upsides to make up for any downsides: Access to all three hazards, Recover, Haze, and Unaware lets it serve as a backup check to effectively any offensive sweeper, especially as Poison gives a fantastic resistance to Fairy, Fighting, and other Poison moves. Clodsire's statline is optimized better than Quag's, letting it invest into Physical Defense fully and become a far better mixed wall. f you hate Rain teams as much as everyone did last gen, then Water Absorb essentially stops all forms of having fun, as they will have to brute force through with +0 Hurricanes or by poking it repeatedly with resisted attacks. What's not to love?