Rain. The most used and according to most people, the most helpfull weather in the pokemon game. Rain boosts the power of water moves by 1.5 and makes thunder and hurricane have 100% accuracy and gives thunder a chance to break through protect/detect(a 30% chance). it also activates the swift swim(doubles speed in rain), rain dish(heals hp in rain),Hydration(heals status condition in rain) and dry skin(heals hp in rain and from water attacks but damages in sun and makes the pokemon weak to fire).
There are two ways to get rain in battle. one is through the drizzle ability and the other is through the rain dance move. There are only two pokemon that can have the drizzle ability which causes it to rain until a new weather is put in play.. they are Kyogre and Politoed so if your making a rain team one of these two is a must.These two pokemon are only usable in the Uber and ou tier so if your making a uu team use rain dance. Rain dance makes it rain for 5 turns but that can be extended to 8 turns if the pokemon who uses rain dance is holding the damp stone.
The pokemon that benefits the most from rain are water types, grass types, electric types,steel types, swift swimmers and Dry skin pokemon. The main pokemon you will need is either Kyogre(Uber) or Politoed(OU). If you cant get one of these or you want a big challenge Use a pokemon with rain dance holding the damp rock. Even if you have a Kyogre or politoed i still recomend having a pokemon with rain dance in case the opponent knocks out your drizzler and sets up their own weather. The next pokemon should be an electric type with a high special attack that knows thunder since thunders accuracy is boosted in rain and to take care of the opponents water types.Grass can also be used to defeat the opposing water type pokemon and take less damage from fire types. That is also why i recommend using a steel type pokemon. There is really only one good pokemonwith dry skinand that is Toxicroak so adding that to the team will help out alot.
The main counter to a rain team is another weather team. Sun is the best counter as it weakens water type moves and strengthen fire moves. Hail can also be effective as a counter weather. Sandstorm is not a good counter since water types can easily knock out rock and ground types witch are two o the types that benefit from sand.
Opposing grass types can be a pain to deal with for a rain team unless one of the water type pokemon knows an ice attack or are part ice