Mantine

Mantine is a great special wall in OU and is capable of walling a significant portion of the metagame. Because of its typing, it's able to be one of the best answers to Volcarona in the tier while also being able to switch into threats such as Ash-Greninja and Mega Charizard Y. Mantine is also a good Defogger, being able to deter Pokemon such as Bisharp and Landorus-T from switching in to take advantage of it with a Scald burn. However, Mantine is very passive and can drain momentum from your team, and its weakness to Stealth Rock is suboptimal for a defensive Pokemon. The prevalence of Electric-types such as Magnezone and Tapu Koko also hinders Mantine, as it can't do much against them. Finally, it has relatively low Defense, even when invested, so Pokemon such as Mega Medicham and Landorus-T can easily do massive damage to it.