Politoed

Politoed seems like a fairly unimpressive Pokemon at first glance, but it has access to a great ability in Drizzle that makes it a crucial Pokemon for OU rain teams. It's not that bad of a Pokemon either, as its defenses are high enough for it to tank some fairly strong hits and set up rain multiple times in a match. Furthermore, it has decent offensive stats, a Speed stat that places it just above Clefable and Azumarill with a little bit of investment, and an outstanding support movepool with several options it can profit from, such as Toxic, Encore, and Perish Song. With the weather nerf in XY, rain's popularity as a playstyle has decreased, but that doesn't mean it cannot still be effective. Politoed's major flaw is its lack of reliable recovery, which limits its supportive capabilities, but Rest is usually enough for this concern to be minor. It also has Grass and Electric weaknesses that are compounded by the Water-type Pokemon that enjoy the rain support it provides, forcing the rest of the team to compensate for this, and it is really only viable on rain teams, but these factors don't stop Politoed from being the single most effective form of support for rain teams.