Feel the need to comment on some posts here. Tldr though is that I think Ludicolo is the best rain mon by a good amount and if we want to nerf the playstyle I feel as though it should be the first target. Arceus and Lucario are both okay and somewhat different from each other. Pikachu is crappy but maybe less crappy now.

Rain sometimes feels pretty strong to me, largely because of Ludicolo which the best rain sweeper by a pretty noticeable margin I think. CB and SD have somewhat different checks sometimes but it's pretty much always Tera Water to just overwhelm everything. It naturally beats bulky waters by not really caring much about water moves at all and just beating them with Grass STAB. Ice Spinner basically rounds out its coverage perfectly but I've also seen Knock Off which can really wear stuff down and Drain Punch which can be annoying if your plan is to wear it down with LO recoil or something. But most of the time it should be running Ice Spinner I think. A really unexplored move I've seen on it is Encore which can force walls to switch out and essentially get you another kill, this can be really effective against like a Skarmory or Brambleghast that clicks a recovery move at the wrong time. Basically this mon is annoying, it's super powerful in rain, very fast even outside of rain and can find a lot of setup opportunities easily. Its main checks are like Toxapex, Alolan Persian, Skarmory and Brambleghast which are really passive and exploitable a lot of the time. Toxapex is probably the most stable against it cause it can threaten back with a poison move but I feel like I don't see it much compared to other bulky waters that offer more role compression like Quaxwell or Alomomola. Basically the most dedicated physical walls are needed to actually check it defensively so usually it's a game of wearing it down with hazards and sometimes LO recoil to put it in priority range while playing smartly with your Koraidon to keep it under control. That or surprising it with a defensive tera. All that said Ludicolo is always the hardest mon for me to deal with on rain and probably the most banworthy thing on that team style

Poliwrath is okay but a little bit overrated to me, it's pretty frail by the standards of this format and I find that it can be relatively easy to sponge with AV Mienshao or Alomomola. It's almost always Specs but can be punished for locking into anything besides a water move in general. It also has a bit more competition with Barraskewda which is better in a lot of ways I think. But Poliwrath is certainly a fine choice for a rain sweeper. Shout out to the one player running it with Belly Drum, that set completely surprised me and swept my team as I went to my spdef pivot. Good set to flip its matchups.

The fact that barraskewda can be physical, special, or in between can really foil checks that focus exclusively on one defense stat. It doesn't have good coverage or secondary STAB like Poliwrath but it doesn't really need it to spam big water attacks. This is maybe the best rain sweeper with Flip Turn which is great for scouting and chipping things down, it has pretty decent bulk for an offensive mon especially compared to most of the other rain sweepers. It probably isn't as good on paper but I've had success with a mixed Mystic Water set with Liquidation and Hydro Pump, this is something that's pretty unique that it can do to prey on AV users and dedicated physical walls depending on the matchup

This is a really good mon too, having good special defense basically fixes its one weakness. My main electric immunity is Vibrava which is not a switch in really, that and Excadrill which generally needs to try and speed creep it and not get beaten down by Body Press, stallers that use Clodsire probably don't care much about it though.

I think iron bundle's a good rain sweeper too, it has a good speed tier one point ahead of koraidon and flutter mane. But it doesn't really have the power to break through some things and will lose a teamslot to Poliwrath a lot of the time which is more hazard resistant and faster with swift swim. Its coverage is really great though so it can break teams that rely on specific resists but it can also just get sponged like Poliwrath sometimes

I think this mon is just worse than Iron Bundle mostly cause it doesn't have freeze dry. Mixed potential is interesting but I think it has too much competition in general and just too many flaws

You see this on rain as well sometimes as a secondary sweeper, even if it shines more as a standalone breaker, Kyogre can still put in a lot of work. Its main flaw is just an awful speed stat and needing to be at full health to retain most of its power, so you usually see it with Scarf and coming out in the early game. At the same time it faces a lot of the same flaws that other choiced special rain sweepers face in this which is that it can be sponged by immense special walls and water resists like AV mienshao, alomomola, pex. All this basically contributes to why I feel like Ludicolo is the best rain sweeper by a lot, because it doesn't have to be choiced so it can set up and get the power it needs to break through things in this meta, and it doesn't have to deal with AV.

I tried using palafin. The main thing to note is that hero and zero form basically have the same stats in this so it doesn't need to come out early to get most of its power. Being a physical rain sweeper for reasons I mentioned before and having strong priority is amazing in some matchups but it unfortunately doesn't really seem viable to me because it just has rubbish speed for a CB user that can be speed crept easily and is actually slower than a ton of stuff, and Wave Crash is just so punishing to click in this since everything has a huge HP stat

I feel the need to mention pelipper because on top of setting rain it actually has good stats on its own, it naturally is a pretty decent switch in to koraidon sometimes and can even pressure stuff with knock off or rain boosted attacks. I think it has more use on its own in this tier than in OU maybe even, good job pelipper.

ekiller is definitely like okay but it really struggles with skarmory like a lot, I think it's viable on offensive team structures mostly and generally does good against most other things. In general I think it appreciates pawmot's absence but whenever I tried using it got shut down by skarmory, dondozo, brambleghast, or some combination of these so I think it needs specific support. I definitely think it's good though.

Definitely good too but it's a lot weaker than arceus with its priority and reliant on fighting STAB so you can usually bring in something like weezing-g or ho-oh with a lot less fear. I think the fact that its immune to Toxic and can get some recovery with Drain Punch, but is also weak to ground, fire and just weaker and slower to start off means that it has different checks than arceus so its not fair to compare them. but arceus is more immediately threatening to more things so i think its overall a better mon in this format. I gotta say that the bad speed stat is really a killer sometimes, it has the same speed stat as Ho-Oh which is just pathetic in this format, it's easily outsped by even a lot of defensive mons (pawmot was a big one, weezing-g and mienshao are relevant too, among others). Sometimes lucario is just so bulky and resistant that it just sets up really freely, and you need to try and go for a scald burn or a wisp or something and it just reveals lum berry or rest and kicks your ass. Sets like that are way more effective against balance teams, but in general lucario's a bit easier to manage I think by brambleghast or phys def ho-oh or deoxys-d.

I've never seen this mon do anything super impressive against any teams with any sort of bulk but it is at least pretty cool and can definitely get the job against frailer offensive mons. It's definitely fringe though. If you're looking for something that just uses priority moves in general I think bisharp or ekiller are probably the best mons for that but persian is probably okay on some teams but I've never been too impressed by it otherwise.

Pikachu has the same bad speed stat I mentioned is so killer for Lucario but has none of the bulk to back it up, and it doesn't really have good priority either. So it is a complete dead weight in a lot of matchups. It is really strong though. With stall teams no longer able to run Pawmot which does pretty solid against it in general, it might be able to do something now. A lot of stalls run Unaware Clodsire which stands a pretty good chance of losing to Surf. It's pretty bad against Brambleghast too though. Tera Dragon Dondozo is a disaster for it as well. And oh, Dugtrio's a thing too.