Im gonna preface my opinion with this
Rain is not broken, but its presence has a very bad effect on the builder
Most teams end up constricted to running Hydrapple + slowking because its the easiest way to prep for rain defensively. Now this isnt to say that this core is bad, and it likely would still be used even if rain was gone,
but the fact that you get shoehorned into into it so often is very exploitable. Many in the builder ive felt like i've been unable to be prepared for less common threats like okidogi or raikou because i find myself locked into running the same cores a lot. There other options for answering rain defensively such as storm drain gastrodon or toxapex, but pex is even easier to bully than slowking and is often bullied by a lot of the same stuff that bullies king, while gastro is kinda just bad in any mu that isnt rain and still manages to be shaky into rain at times.
This also only really goes over the effect of standard rain with peli barra zap drill apple filler. Rain has other options to mess around with. For example, if you put a floatzel over the barra for wave crash, you end up with funny calcs like this.
252+ Atk Choice Band Tera Water Floatzel Wave Crash vs. 252 HP / 252+ Def Slowking in Rain: 232-273 (58.8 - 69.2%) -- guaranteed 2HKO
(Nice max defense now check this out)
252+ Atk Choice Band Tera Water Floatzel Ice Spinner vs. 248 HP / 96 Def Hydrapple: 416-492 (100.2 - 118.5%) -- guaranteed OHKO
Of course you can run both of them together to have slowking scout and then pivot to apple, but all doing this does is further the point that teambuilding with rain present is kinda fucked. Not to mention i didnt even get into options like azu to bully the slow water resists or triple axel quaq for that real good wack shit.
Other workarounds
The obvious one is using water resistant teras to try and withstand a rain assault and fire back with smth like rhyperior. However this runs into the awkward issue of trying to bait the swift swimmer with something that isnt ohkod by flip but is ohkoed by liquid, as if both kill then its just gonna flip turn, and if neither kill its still gonna flip turn. Its also not exactly you get to automatically pick smth vs the rain team as its not like rain teams are defesively inept. Using the rhyp example, if barra flip turns on you as you tera dragon, they can just go to hydrapple to scare you off, and now your rhyp has to contend with both not being able to help dance around rain zap by drawing out weather balls, as well as the fact that it now the barra just uses you as flip fodder like everything else.
Out-offensing it is another option ive seen brought up, however this ends up requiring very specific positioning to prevent the swift swimmer from getting in and clicking a free flip/liquid/whatever depending on the situation. For example, one mon that is brought up as smth to out offense rain is latios, leading it as the rain leads pelliper and clicking a button. However as mentioned earlier, rain isnt exaclty defensively inept, and typically as drill and apple to help play around a situation like this (in this case, drill fends off the latios, and then the rain threats give latios a big pain getting back in). This makes it a lot harder than it looks for latios to click buttons in the rain mu. Kix can be a pain for rain, but requires a sack or for slowking to get in without getting pivoted on, and as mentioned earlier needing slowking this much is exploitable.
Another potential workaround is to outplay telegraphed moves, but to be completely honest playing against rain can be just as telegraphed as using it. Barra gets in and you have gastro as your answer, pretty obvious whats gonna happen here so the barra uses banded cc to clock the gastro, so to me the whole "outplaying" idea is kinda crappy bc rain can do the outplaying too.
Messing with the weather
Already went over slowking a good bit and both ninetales sucks so this is just gonna be the sand guys
Really doesnt like switching in to the attacks from rain, even zap as that still stings a good bit and getting volted on is just the worst, so it runs into the same issue kix does of wanting sacks or to be pivoted in. Its not really a BAD workaround but it isnt exactly the greatest considering its janky mu into all the rain mons (also in my opinion this mon is kinda on a down turn due to the rise of coba while also being kinda bad in the event a kommo shows up, but kommo discussion is for another day so ill just leave it there.)
Significantly better at messing with rain if zap comes in given its elec immunity and the sand changing weather ball to rock, however its not exactly easy to fit as it easily lets in stuff like latios and apple. Its not a BAD mon but isnt exactly that great or splashable either and its overall usage reflects this, as its overall quite slow paced compared to a lot of other stuff and ends up being a tad exploitable as a result.
The Centralization of Rain
One could argue that rain brings a centralizing force to the tier by limiting the amount of random wack mons show up and try to gimp games. However, id have to disagree that its a good force here, as i personally find rain to make the issue of random wack mons worse. Rain in of itself is not its own mon like other centralizing forces such as zapdos or excadrill, It isnt smth i can put on a team to help blanket check a bunch of stuff
because rain in of itself is the team. You cannot prep for stuff WITH rain by putting it on a team in the same vain as zapdos or excadrill, there is only prepping FOR rain. To me, this is a sign of a negative centralizing force.
In terms of prepping FOR it, I view it similarly to garganacl, its smth that if super prepped for, sure it kinda just flops. But if not super hyper prepped for it can very easily find a way to eat you, either by exploiting the common workarounds to it like apple + king stuff or by it just being sent into an opponent who was less prepped for it bc they wanted to be safer into the overall meta. Of course its not to the same degree as garg, apple king and pex arent bad like reun and covert cloak were (However you could theoretically argue the counterplay to garg was actually more vast than rain due to cloak being an item but you get the idea). However i must repeat that being forced into the same stuff a lot is not a good thing.
One could say that people need to get more creative, but to be real that could be said for anything. Bulkier apple is a creative set for checking boulder, covert cloak (insert ghost here) is a creative way to beat garg, etc etc. Ofc theres a limit to what is generally acceptible here, such as a PU shitmon like palaosand being used for boulder, but uh... storm drain gastro isnt that much of an improvement to me and here we are usin it so... yeah.
Conclusion
I believe rain should be banned, its overall just a negative centralizing force that limits what people can do in the builder by jamming up defensive cores with its presence. To me, it forces them to be built in a way that ends up being pretty easily exploited by less common set up threats as by attempting to stray away from the norm, your mu into rain can be compromised pretty easily. Not to mention the fact that rain can also mix itself up to mess with the usual workarounds. Overall rain is a bad presence in the tier, and i will be voting
ban.