Kingdra

Kingdra's decent Speed stat in combination with Swift Swim allows it to outspeed the entire unboosted metagame, as well as most Choice Scarf users, when in the rain. This combined with a decent Special Attack stat makes it a top Swift Swim user in the metagame. Kingdra's access to a powerful secondary STAB move in Draco Meteor is a blessing, giving it the means to pressure or KO Pokemon that annoy rain teams such as Mega Venusaur and Mega Latias. Kingdra's Dragon / Water typing is also useful for checking Ash-Greninja as it is a rain attacker that isn't prone to Water Shuriken. Unfortunately, bulky Water- and Grass-types such as Toxapex, Ferrothorn, and Assault Vest Tangrowth are some of the most common defensive Pokemon in OU, meaning that Kingdra will have trouble sweeping most teams. Kingdra is also heavily dependent on rain in order to be effective, as it is relatively weak and slow for an offensive Pokemon outside of it.