I'm here to break up multiple topics. Starting with...
COUNTERING TOXAPEX
Honestly, Pyukumyuku deserves the spot for the worst enemy of toxapex. It learns soak and toxic (among other stuff) and poison/steel types arent built to handle poison. But that all goes away when Pyukumyuku uses Soak, then it can use Toxic on it and do damage over time. In terms of raw tankiness, Toxapex does deserve the higher spot, but Pyukumyuku has very good base Def and Sp. Def, with 130 at each stat. Assuming Soak doesnt get banned or something, Pyukumyuku looks to be an UU Steel-Type/Poison-Type Tank Killer.
TOXAPEX ITSELF
Toxapex makes it's home in OU, and it won't budge from there, though. It's tankiness is a force to be reckoned with, and it gains access to amazing moves, like Recover, Toxic Spikes, and some other moves.
ABILITIES OF TOXAPEX
Toxapex has 3 abilities:
Merciless: The Pokémon's attacks become critical hits if the target is poisoned.
Limber: Its limber body protects the Pokémon from paralysis.
Regenerator: Restores a little HP when withdrawn from battle.
Does high damage on one specific thing: Poison. WHO CARES?! It's a defensive pokemon, and isn't meant to deal damage!
It can't get paralyzed, but that's useless. Nothing more, nothing less. Useless. Electric still does super-effective on it.
This is where it gets more insane than FilthyFrank (hey b0ss). REGENERATOR + RECOVER. You can switch out when low HP, and take down the threat using a different mon. Then u switch back in, regenerate, and recover. Your probably gonna be at full HP, depending on how much damage you took first.
Toxapex has one thing going for it and rare are the mons that share it in the game: it just never dies.
It's an incredible defensive Pokémon and many, MANY would kill to just at least get Recover, but this thing gets BOTH Recover and Regenerator!
It's so passive though. Beside Toxic damage, it's offensive presence is abysmal, but at least it is not set-up fodder because it has Haze. Oh yea did I mention it has Haze too? It just has everything for a defensive mon, but unlike ones like Ferrothorn, it just sits there and eats attack all day without being able to dish out decent damage.