I'm gonna say this, rain isn't a great play style in OU since everyone and their mothers love their bulky waters and grasses and thus, have a rather tough match-up. However, that doesn't mean that it isn't viable, it's just it's not the easiest play style to get that consistent of results so I'll give some tips to get some consistent results on it.
First off, the name of the game with rain is to be as offensive as possible. You cannot be defensive with a rain team, or any weather team for that matter. Yes, weather teams love their defensive backbones but they should still play very aggressively. To that, you need to drop Goodra, Nidoking and Heliolisk.
So first, use a defensive Pelipper. You need this to be bulky to setup rain multiple times. As your weather setter, you shouldn't use this as an offensive mon but rather, use its second greatest advantage over Politoed as a rain summoner, a pivot move (U-Turn).
On Barraskewda, just use max attack and max speed. This thing is never gonna survive any hit at all so you don't need the bulk. Barraskewda has a lot of move-slot issues as it wants to get rid of all the bulky waters. Fortunately, it has both crunch, Drill run and Psychic Fangs for Toxapex and Slowbro. Just whatever you do, never have this take on Ferrothorn on its own. Taking on opposing Ferrothorn should be a teammate's job and not Barraskewda.
With Heliolisk gone, you need Ferrothorn. This is non-negotiable. You need Ferrothorn for a couple of reasons, one its hated fire weakness gets cut in half, meaning it can feasibly stay in against a fire type and annoy them with whatever annoying moves it has. The second reason is Rillaboom. Rain teams are bananas. Gorillas eat bananas so we use the durian to ward off the gorilla. It's not gonna 100% take on Rilla on its own but I'll expand on that later.
On Swampert, just drop Brick break and Rain dance. The idea with rain is that you wanna abuse the rain without relying too much on it. The same also goes for every other weather but whatever. Replace those two moves with Flip Turn and Earthquake or Protect, depending on what you prefer. Just keep Flip turn. It's a great move. I would also suggest going for a specially defensive spread as Swampert is one of the ground types that doesn't fear one of rain's greatest enemies, Rotom-Wash. Not only that, it can actually threaten it with Toxic, making it easier to deal with.
Now, I did say that Ferrothorn should not handle RIllaboom all on its own. Yes, it stuffs banded ones but can lose to Swords Dance Superpower, as such, you will need an offensive answer to Rillaboom. Fortunately, its offensive answers just love the rain. There are a ton of options for this slot. You have Nasty Plot Tornadus Therian, Zapdos and even Moltres. Basically, any bird that has hurricane should be on this slot to offensively check Rillaboom. The key word here is Hurricane as you need to kill Rillaboom at all costs otherwise it's just gonna glide like crazy. Aside from Rillaboom, the birds should also deal with opposing Ferrothorns. Yes, it is neutral to hurricane but it is not gonna want to eat that many repeated hurricanes especially ones that never miss.
For this last slot, you have a lot of options and probably the only slot that has a lot of flexibility. You can choose here from Thunder users to fighting types or mons with strong water type moves so you can use the rain to power them up.
https://pokepast.es/6bbce270551cc7b6
In that paste, these are the mons that should be on the team regardless of what happens for the reasons mentioned earlier.
https://pokepast.es/08db74efe367f995
For the fifth slot, choose one among all these bird options and I also mentioned earlier why you absolutely need a bird.
https://pokepast.es/3576f5835cb2e511
For the last slot, choose one among all of these options. This last slot serves to be a rain abuser that doesn't exactly need rain to be a threat.
Note, that all of those are just sample sets and you can tweak them however you like. So you're final team should consist of rain core, one of the birds and one of the rain options.
