Politoed (M) @Damp Rock (Tera: Ice)
Ability: Drizzle
EVs: 108 HP / 200 SpA / 140 Def / 60 SpD
Modest (+SpA -Atk) Nature
-Weather Ball
-Icy Wind
-Helping Hand / Perish Song
-Protect
My favorite little buddy, Politoed :D This guy is a beast.
The main time you want to play him is to set up a rain team on the first turn. Keep him alive and switch him out so he can set up the rain twice if necessary.
He is a tank and hits like a dump truck with Weather Ball in the rain. This can OHKO many, many Pokémon.
Icy Wind is very important in double battles for speed control.
Ice Tera is to boost the damage of the Icy Wind, and to help counter grass types. Having a Tera Type that is usually considered a bad type is fine here, because water is a very good type, and you obviously aren't forced to Terrastallize.
For me, Helping Hand has been awesome. Many people run Perish Song (which I will likely never run because I like to keep Politoed around to spam Icy Wind often times).
Finally is Protect, the most overpowered move in the game. If you suspect your Politoed will faint, you can Protect and switch out to be able to set up rain again later if needed.
Some people may say that Leftovers is more important than the extra 3 turns of rain with Damp Rock, but those 3 turns can be VERY clutch,and this lets you use Leftovers on another Pokémon.
The EVs being defensive are important so Politoed can stick around and be a pain for a long time, but I've given it a lot of attack to make it a big enough threat.
Pairing up Politoed with a very powerful Pokémon that has Swift Swim is very good since the powerful one (like Ludicolo @ Life Orb, for example) can deal huge damage at doubled speed while Politoed uses Helping Hand for a boost to your ally, Icy Wind for speed control, and Weather Ball for extra damage. Your opponent will be forced to remove the powerful Pokémon (or else it will simply sweep them), and if they do that, you can just throw in another (like maybe a Zapdos that knows Thunder and Hurricane), all the while Politoed is weakening them. And these are just a few powerful Pokémon that pair with Politoed.
Thank you for reading all the way down here for the full explanation, and I hope you win many games with the GOAT, Politoed.