Toxapex. Go ask ten random competitive Singles players which Pokemon they would delete from existence Thanos-style if they could pick only one, and I guarantee you at least eight pick this sea demon. Toxapex is a case study in overtuning; whether it was intentional or not, a perfect storm of factors come together to form possibly the overall best defensive Pokemon in the game, and an absolute nightmare that many teams can have a ton of trouble putting down.
The first factor to look at when examining Toxapex is its stats. Its offensive stats are pretty pathetic, its Speed is terrible and its HP isn't anything to write home about... and then you see its defenses. Titanic 152/142 defenses put even most bulky legendaries to shame, more than making up for Toxapex's mediocre HP stat and allowing it to shrug off all but the most powerful hits. Its typing aids in this endeavor as well; while it leaves it vulnerable to Ground, Psychic, and Electric, it packs resistances to many other common types like Fire, Fairy, Fighting, Bug, Ice, and Water. It also absorbs Toxic Spikes, which is nice for the more defensive teams Toxapex is almost mandatory on.
The thing is, while Toxapex might be bulky, it isn't
the bulkiest Pokemon in the game by metric of raw stats, so why does it have such a reputation as an unkillable abomination? That's where its hidden ability, Regenerator, comes into play. Regenerator allows the user to recover 33% of its maximum HP when it switches out (this includes being forced out by moves like Roar, or a Red Card). This ability to passively heal while switching out to a safe check is a huge part of what makes Toxapex so hard to pressure. Most defensive Pokemon must be cautious about taking unnecessary chip damage or switching into Pokemon that could potentially have super-effective coverage, lest they be put into the KO range of powerful sweepers on the opposing team. Regenerator significantly ameliorates this concern.
Let's say your opponent has a Dragapult on the field and you decide to switch into your Toxapex. Usually this is fairly safe play as Toxapex can shrug off even two consecutive Draco Meteors, or even better, eat the first and then switch out to a Fairy or Steel teammate on the second and recover some HP with Regenerator in the process. However, surprise, this Dragapult has Thunderbolt and just took 60% of your Toxapex's HP away. For most defensive Pokemon, this is a really bad spot. They can't stay in as they'll just get KOed, and even if they switch out, their wall is stuck at 40% of its max HP. If there was a Pokemon in the back that you were relying on it to check, say a Volcarona, it's probably not going to be able to check that threat anymore.
Regenerator massively reduces how much misplays like this punish you. Now you can switch out of Dragapult's second Thunderbolt and be at 73% HP instead of 40%, giving you more than enough room to take a hit from most Pokemon you'd want to check in the back. You can also freely throw Toxapex in against Pokemon it checks, and even if you get forced out again immediately, that's fine. You profited 33% HP in the process. This massively skews the risk/reward factor of most plays you might make with the Regenerator Pokemon and lets you play a lot more recklessly with it without getting punished.
Okay, so Toxapex is hard to kill. At least it's passive, right? Those offenses aren't doing any significant damage, and it lost Scald and Knock Off in the transition to Gen 9. Well, no, not really. The loss of Scald does make it quite a bit safer to switch in on Toxapex, but it still has Haze to prevent attempts to use it as set-up bait, it still has Toxic to put opponents on a timer, and it has Chilling Water to prevent physical opponents from making progress against it. While it's no offensive powerhouse, it's not fair to call Toxapex passive either as it certainly isn't. It just has other means of forcing progress (and preventing the opponent from doing the same) than raw damage output.
All of these factors create an incredibly hyper-tuned defensive Pokemon that can be incredibly frustrating to deal with for many team archetypes, and it's no wonder Toxapex finds itself as a staple on pretty much every team archetype outside of hyper offense.
Toxapex @ Black Sludge
Tera Type: Fairy, Steel
Calm - Regenerator
248 HP, 252 Sp. Def, 8 Defense
-Chilling Water / Sludge Bomb
-Haze
-Recover
-Toxic / Toxic Spikes
This version of Toxapex heavily invests in special bulk to act as a more reliable check to the likes of Volcarona, Dragapult, Gholdengo, etc. Moving some EVs into Defense is a fine option as well; it really depends on the rest of your team. As for the moveset, there's really not much to say. Chilling Water does at least a bit of damage and drops the opponent's Attack stat, watering down powerful physical wallbreakers and making their attacks easier for Toxapex to take. Haze prevents attempts to set up on Toxapex. Recover keeps you healthy without forcing you to switch out constantly, and Toxic puts opponents on a timer and is your primary means of actually doing damage. Toxic Spikes can also be considered if it fits onto your team better, but Toxapex rarely has room for both. Sludge Bomb is another move I've experimented with; the high chance to inflict standard Poison can be nice to inflict some extra chip damage on opponents that switch into Toxapex, but it also means you can't inflict them with Toxic poison. It also means you let in Steels such as Kingambit completely for free, which isn't great in a metagame where Kingambit is one of the scariest late-game sweepers available.
Toxapex @ Assault Vest
Tera Type: Fairy, Steel, Fighting
Modest - Regenerator
248 HP, 252 Sp. Atk, 8 Sp. Def
-Surf
-Sludge Bomb
-Infestation
-Acid Spray / Ice Beam / Tera Blast
Okay, I know what you're thinking. No way is that a real set that competitive players actually use seriously. I thought the same thing when I first heard about it. Well, I regret to inform you that sadly, it's real. The current Singles metagame is very biased in favor of heavily offensive playstyles, which happen to be the one playstyle that Toxapex usually doesn't fit in on. You'd think this would mean Toxapex usage would subside and we might get some reprieve from the unending nightmare of this thing's existence, but no. Toxapex adapts and overcomes. This offensive Assault Vest set has seen some usage in high-level tour games as a defensive pivot that can dish out some surprising damage to opponents that expect a more passive version of Toxapex. Opponents that Assault Vest Toxapex can beat 1v1 include Walking Wake, Volcarona, Dragapult, Azumarill and Iron Valiant, and it accomplishes this with a lot less room to exploit than more defensive Toxapex variants. The downside is it must switch out to recover HP, which does hurt its longevity significantly in longer matches. Fighting Tera + Tera Blast in the last slot is a tech for luring Kingambit, who usually sees Toxapex as set-up bait. It does leave you with a much less optimal defensive typing for the rest of the match, however.
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