Firstly, thanks for making this competition! I had a ton of fun making this team, and I'm quite happy with what I came up with.
Now, without further ado, I present to you, Rain's Rain Team:
PokePaste:
https://pokepast.es/192050dfc0f6c39b
Team Breakdown:
Politoed:
Politoed is here to set up rain with its Drizzle ability and then assist its allies by using Helping Hand to boost the power of allies' attacks or Icy Wind to reduce the enemies' speed. The team has both+ fast and slow attackers, so the speed control may or may not be necessary depending on which mons were brought and what the opponent is doing. In the event where it is not needed, Scald is an option to provide some decent damage and possibly a burn. The last move is Protect because Protect is good. Politoed's item is a Damp Rock to extend the duration of rain, and its EVs are specially defensive, which might help it to survive a Thunderbolt here and there. Politoed cannot do much on its own, but it plays a pivotal role on the team by setting up rain, which the rest of the team is poised to abuse.
Celesteela & Rhyperior:
I found this to be a very powerful pairing, but Celesteela is the real star of the show on this team. It has very good bulk and only two weaknesses, both of which can be completely covered up by its allies. It's Fire weakness is effectively eliminated due to the fact that rain halves the power or Fire moves, and it doesn't need to worry about Electric attacks at all due to Rhyperior redirecting them with its Lightning Rod ability. Its considerable offensive strength and proper support can turn it into an unstoppable juggernaut while Dynamaxed. Rhyperior can make quick work of mons such as Torkoal and Regieleki with its STAB Earthquake, and in turn, Celesteela takes out many of Rhyperior's potential threats, such as Rillaboom, with its STAB Max Airstream. The rest of the moves are fairly standard; I chose a physical set for Celesteela, but a special set would probably work almost as well. Celesteela's EVs are focused on Attack and HP to improve its damage and bulk respectively; Rhyperior has done the same, except it invested in its somewhat poor Special Defense instead of HP. Celesteela holds a Weakness Policy (more on that later) and Rhyperior holds a Focus Sash, which allows it to survive a 4x effective hit and fire off one last attack that the opponent was likely not expecting. The only things this duo of colossi fear are strong water attacks, which is somewhat ironic due to their presence on a rain team. However, this is not a problem at all, thanks to the next member of our team.
Regieleki:
Regieleki's role on this team is very simple - to nuke everything with 100% accurate STAB Thunder. It does this job very well thanks to its blistering speed and Transistor ability. It has a Timid nature and EVs fully invested into Special Attack and Speed, and it holds a Magnet to further boost the power of its Electric moves, making them powerful enough that they can even take out foes that resist them! In addition to Thunder, Regieleki has Electroweb for more speed control and Volt Switch to pivot out while doing damage. The final move is, of course, Protect. It's important to note that while Regieleki does an excellent job of covering up Rhyperior's 4x water weakness, it should not be on the field at the same time as Rhyperior. Although it may seem to be a glaring counter-synergy, it is actually surprisingly easy to work around while playing, and will virtually never be a problem as long as you are careful.
Incineroar:
Of course I had to include Incineroar; it's not without reason that he is present on nearly every VGC team. My Incineroar set had one slot taken up by a more unusual move choice, however - Ember. This is here to activate Celesteela's Weakness Policy while dealing minimal damage to it. The other moves are standard support: Fake Out, Snarl, Parting Shot. It doesn't have Flare Blitz because its effectiveness would be reduced by rain, and it doesn't have Darkest Lariat just because that slot was taken up by Ember, which is vital because the Celesteela/Rhyperior duo is so good at covering its own weaknesses that the WP doesn't usually get activated by enemies. EVs and Nature are configured to give it good physical and special bulk, and it holds a Sitrus Berry to stay alive longer. Similar to Politoed, it's role on the team is vital, subtle though it may be.
Kingdra:
I wasn't originally planning to bring Kingdra, but as we all know, it synergizes incredibly well with Politoed and rain, so I decided to run it after all as a secondary Dynamax option. Kingdra has the standard Life Orb set with Hurricane (100% accurate in rain), Scald, and Draco Meteor. It has all its EVs invested into Special Attack and Speed to make it hit as hard as possible, and a Modest nature because I'd rather have more damage over more speed. Ability is of course Swift Swim, which will make it fast enough that a Timid Nature isn't necessary. Last move slot is Protect to help it stay alive longer. It's a very good option for an attacker in the event that the Celesteela/Rhyperior core isn't suitable, and although I call it a fallback option, it isn't by any means a downgrade. Kingdra is also very good against Sun teams, which the rest of the team tends to struggle against. In the event that the enemy sets up sun, you can easily turn the tables with a Max Geyser from Kingdra, hopefully killing the sun setter in the process.
Playstyle:
As a whole, the team is pretty balanced, with a mixture of glass cannons and bulky attackers. It can hold its own inside of TR, and even against enemy Tailwind. Like most rain teams, it struggles a bit against sun teams, but the disadvantage is not so large that it can't be surmounted. The general strategy is to lead with Politoed and Kingdra, Eleki, or Incineroar. The former two provide a good offensive pressure, and the latter can take the edge away from aggressive enemy teams. Often, the opponent will predictably target Politoed with a Grass attack, which makes for the perfect opportunity to safely switch into Celesteela. When you switch into Steela, you will generally want to have either Incineroar (for WP activation) or Rhyperior (for attack redirection) as a partner. If you didn't Max Kingdra, then you will usually want to Dynamax Celesteela as soon as possible to start stacking up boosts with Airstream, Knuckle, and of course its ability Beast Boost (its EVs are such that it will receive an Attack boost after KOing a foe). If you have Rhyperior as a partner, then you can take advantage of Celesteela's Max Knuckle to fire off some powerful Earthquakes and pick up any KOs that Celesteela missed. The more defensive alternative is to keep Incineroar in, using Snarl and Parting shot to neutralize potential threats to Celesteela. Once Celesteela goes down, it is probably time to switch into your secondar attacker (Eleki or Kingdra) and sweep through the remains of your enemy's team.
Replays:
Against another rain team. Rhyperior's Sash was essential to winning here. (I accidentally used an older version of my team which had Kartana instead of Incineroar, but that doesn't matter because I didn't bring it and all of the movesets are the same.)
https://replay.pokemonshowdown.com/gen8vgc2021series9-1326914948
In this match, a WP-boosted Celesteela singlehandedly smashed through the entire enemy team with its powerful Max Moves.
https://replay.pokemonshowdown.com/gen8vgc2021series9-1326997426
In this match, the Celesteela and Rhyperior duo worked exactly as intended Rhyperior redirected super-effective moves and finished off weakened foes, while Celesteela dealt out provided boosts with its Max Moves.
https://replay.pokemonshowdown.com/gen8vgc2021series9-1327182085
Against a sun team. No time to write anymore description because the submission deadline is in 2 minutes!
https://replay.pokemonshowdown.com/gen8vgc2021series9-1327183013
Thanks for reading!
Now, I will admit that Urshifu brings nothing of value to a rain team, but...it's urshifu. A strong balance team killer, which can pose a deadly threat to an offensively oriented rain team Also, it's fairy weakness is coutneracted with the fact that metagross is on the team. 
