Gastrodon

By virtue of Gastrodon's modest defenses, solid defensive typing that leaves it with only one weakness, and access to a reliable recovery move, it finds its niche in OU as an answer to special attackers like Heatran, Tapu Koko, Greninja, Magearna, and Mega Diancie. However, some of these Pokemon such as Heatran and Tapu Koko possess ways of getting around Gastrodon. While it being a Ground-type that isn't weak to Hidden Power Ice is already a great boon, Storm Drain only adds to its defensive qualities, making it a great answer to Water-types in general, as well as to rain sweepers like Kingdra and Ash-Greninja. Gastrodon faces competition from Toxapex, which is another bulky Water-type also capable of checking Ash-Greninja and Kingdra and providing Toxic Spikes or Knock Off support. Furthermore, Toxapex also has better overall bulk and access to Regenerator, which makes it less of a momentum drain than Gastrodon. It also struggles with mediocre bulk, especially physically, which makes it difficult for it to take hits from anything besides the special threats it checks.