Barraskewda is a great swift swim sweeper. It’s got a high attack stat as well as speed that’s already incredible without swift swim. It also has a great physical movepool.
Barraskewda @ Choice Band
EVs: 4 SpD/252 Atk/252 Speed
Adamant Nature
Ability: Swift Swim
Liquidation
Close Combat
Poison Jab
Psychic Fangs
Pelliper is the best OU rain setter. It has Defog as well as Roost and a Hurricane with 100 per cent accuracy in rain.
Pelliper @ Damp Rock
EVs: 248 HP/252 Def/8 SpD
Bold Nature
Roost
Defog
U-Turn/Scald/Hurricane
U-Turn/Scald/Hurricane
Zapdos is also a great weather sweeper. It has two STABs with 110 BP and 100 per cent accuracy in rain (Thunder and Hurricane) as well as Weather Ball to hit rock types.
Zapdos @ Life Orb
EVs: 4 SpD/252 SpAtk/252 Speed
Timid Nature
Weather Ball
Thunder
Hurricane
Roost/Defog
Kingdra is a great special rain sweeper. It’s lower tier, being NUBL, but still good in OU.
Kingdra @ Life Orb/Choice Specs
EVs: 4 SpD/252 SpAtk/252 Speed
Modest Nature
Surf
Draco Meteor
Flip Turn
Hurricane
You can also run physical with Dragon Dance:
Kingdra @ Life Orb
EVs: 4 SpD/252 Atk/252 Speed
Adamant Nature
Dragon Dance
Liquidation
Outrage
Iron Head
Ludicolo is also decent (although not quite as good). It’s good as a tank or a special sweeper.
Tank:
Ludicolo @ Leftovers
EVs: 252 HP/252 Def/4 SpD
Bold Nature
Leech Seed
Giga Drain
Scald
Protect/Ice Beam
Sweeper:
Ludicolo @ Life Orb
EVs: 4 SpD/252 SpAtk/252 Speed
Modest Nature
Scald/Weather Ball
Energy Ball/Giga Drain
Ice Beam
Focus Blast
Ferrothorn is a great Pokemon in rain because it’s 4X Fire weakness is reduced.
Ferrothorn @ Leftovers
EVs: 252 HP/252 SpD/4 Atk
Careful Nature
Spikes
Stealth Rock/Leech Seed/Gyro Ball/Power Whip/Knock Off
Stealth Rock/Leech Seed/Gyro Ball/Power Whip/Knock Off
Stealth Rock/Leech Seed/Gyro Ball/Power Whip/Knock Off
Politoad isn’t as good as Pelliper to set rain, but it can work in lower tiers.
Politoad @ Damp Rock
EVs: 252 HP/252 Def/4 SpD
Bold Nature
Scald
Toxic
Rest
Sleep Talk
That’s the best rain Pokemon I can think of.
Hope I helped!