Toxapex

Toxapex is unlike its fellow contenders for the best Pokemon in the tier; what it lacks in set diversity and flashy options, it makes up with its sheer streamlined efficiency in soaking up attacks from huge threats and sinisterly tiring out the opposing team with its bag of tricks. Its defensive stats are gargantuan, and the longevity that it gets from Regenerator is absurd; babysitting threats like Ash-Greninja, Mega Charizard Y, and Volcarona is second nature to it. It's bulky enough to dare opposing pivots like Magearna and Rotom-W to tickle it with Volt Switch while it spreads status easier than hot butter. Being both the best Toxic Spikes setter and by far the most splashable Poison-type in the tier means that if you are interested in not losing to Toxic Spikes or would like to inflict them yourself, you typically have to go through Toxapex. It's very hard to justify not using this behemoth on anything south of offense. However, as many tricks as it may have, it is still just like your average starfish; it is very slow, can hardly lift an arm to deal any significant damage, and is stuck to the ground in ways that wallbreakers like Landorus-T, Mega Swampert, and Excadrill can appreciate. It isn't able to stop apex predators like Mega Alakazam, Swords Dance Gliscor, and defensive Heatran from finding easy entry on it, and there are plenty of other wallbreakers like Tapu Lele, Mega Medicham, and Kyurem-B that it needs to step aside from if they're able to get in unscathed. Lastly, although Toxapex has a plethora of useful options, it can only run four of them at once, so it can be difficult to fit everything you want on it.