Gastrodon

Gastrodon holds an important niche in BW OU as one of the most reliable counters to offensive rain teams. Not only does its ability Storm Drain grant it a Water immunity, but its typing makes it one of the most reliable defensive checks to Rotom-W and Thundurus-T, provided that the latter isn't running Grass Knot. Gastrodon is also deceptively difficult to switch into, as most Pokemon that can reliably switch into Scald are ruined by Toxic. Gastrodon can also check physical attackers like Tyranitar and Gliscor. Finally, Recover and immunity to sand give it an easy time staying healthy to check the Pokemon it is supposed to. With these traits, Gastrodon is often used on sand teams that need a reliable answer to offensive rain teams. However, Gastrodon is very passive, meaning that Grass-types such as Ferrothorn and Breloom can freely switch in and use it as setup fodder. Gastrodon's susceptibility to entry hazards is also problematic, preventing it from checking very strong physical attackers such as Dragonite. Its vulnerability to Toxic also hurts against Pokemon it can otherwise check that run the move, like Heatran and Keldeo. Finally, Gastrodon faces significant competition as an answer to rain teams with Jellicent and Seismitoad, as the former can better check Keldeo, spinblocks, and has a better matchup against Ferrothorn, while the latter does a better job checking Rotom-W with its access to Refresh and better supports its team, able to run Stealth Rock and Knock Off.