Kingdra

Thanks to Swift Swim, Kingdra can outpace the whole unboosted metagame under the rain. Furthermore, Kingdra's decent offensive stats turn it into a great wallbreaker and sweeper, which is amplified by the fact that its Water-type STAB moves are bolstered even further by the rain, and it can also choose to run a mixed set effectively due to its decent Attack. Kingdra also does not stack weaknesses with the rest of its team because it is neutral to Grass- and Electric-type attacks, unlike other Swift Swim Pokemon such as Mega Swampert and Omastar. However, Kingdra has somewhat underwhelming bulk, rendering it susceptible to strong priority moves such as Talonflame's Brave Bird and Bisharp's Sucker Punch when weakened. Moreover, rain only lasts for a maximum of eight turns, and Kingdra's reliance on rain for Speed and power makes it easily exploitable outside of it. Kingdra's Special Attack really isn't that great either, especially compared to other Draco Meteor users such as Latios.