Ludicolo

Ludicolo is the definition of a Rain Dance sweeper in the NU tier. With access to Swift Swim, which doubles its Speed under rain, it can easily sweep teams late-game, especially offensive ones, which tend to be more fragile. On top of that, Ludicolo sports an amazing secondary typing in Grass, which greatly helps it deal with other Water-types that could switch into it, like Lanturn, Poliwrath, and Prinplup. Finally, its good special bulk coupled with a decent defensive typing means that it can also act as a soft check to some Pokemon and even set up Rain Dance on them, including Steelix, Lilligant, and Samurott lacking Megahorn. Unfortunately, while Ludicolo is incredibly fast under rain, in a weatherless situation, it can only dream of outspeeding common threats due to it sitting in a bad Speed tier for an offensive Pokemon, Ludicolo might have trouble setting up Rain Dance against offensively inclined teams, usually relying on support given by its teammates to work properly. As if that weren't already bad enough, it struggles to get past common Pokemon such as Mantine, Ferroseed, and Abomasnow, with the first two not minding Ludicolo's assaults at all and the latter even being able to cancel rain.