Standard RegenVest

This comes in on pretty much every special attacker, because special Ground-, Fire-, and Fighting-types are pretty uncommon, and as coverage you'll mostly be seeing the relatively weak Torch Song and Secret Sword. The moves can be tweaked a ton depending on how you want the set to interact with opposing RegenVest, Imposter, Take Heart Arceus, and various Flutter Mane variants. Regarding most other stuff it doesn't really matter what you run because you either beat them or lose to them. Flip Turn and Volt Switch could be used to imposter-proof pivoting via Desolate Land mon (typically Mega Blaziken) or Ground-type (typically Mega Garchomp), but this requires running a substandard, passive set unable to do anything into said Blaziken or Garchomp, so it hasn't seen widespread use.

Rapid Spin is okay, but it's not recommended to be too reliant on it because every bulky Ghost-type effortlessly spinblocks you. Revelation Dance beats Normalize Flutter, while Gigaton beats other Flutter variants like Flare Boost. I listed Nuzzle first cause it's just nice to have against stuff like Etern and MG Ghostceus.

Mortal Spin should only be run alongside Rapid Spin, otherwise getting spinblocked by Imposter, not to mention opposing Steels, is really bad.

-trollkastel seal of approval

I shouldn't have to tell you how to wall special attackers with this thing, a 5 year old could do that, it comes naturally once you learn the standard sets for each mon. The important interactions are vs opposing RegenVest and Imposter. Often you'll have the opportunity to (for example) allow Knock in exchange for Stone Axe, and this is where you have to actually start thinking about whether this is a good positional decision. In general, Imposter recovers extremely freely on you cause they get your base 150 defenses without being stuck with the low HP. They don't appreciate Knock, Nuzzle, or Salt Cure, but they can also throw all those same things back at you, so it's a double-edged affair.

When thinking about Imposter, there's a lot of subtlety in which tools you want to give to both sides. The more your other mons rely on Imposter (and other RegenVests) taking Knock/para, the better it becomes to run Knock Off/Nuzzle, but this in turn allows Imposter more headway into your own Registeel and the rest of your team. It's highly recommended to run a Knock Off switchin like Plate Arceus, not just against Imposter but in general because Registeel doesn't like opposing Knock users.

Finally, you take very little from Take Heart Arceus save for the rare Secret Sword variants, but they can try to set up on you which does kill you. To deal with this, you can use the very defensive Circle Throw, or hit them with Knock and/or Salt Cure before going to a mon that exploits that. The typical examples of this are Imposter and Scales/Unaware Leftovers TH mons, which can bully opposing TH Arceus once it loses its Leftovers advantage. However, there's nothing wrong with instead using a more general counter like utility Flutter or anti-setup Eternatus and overwhelming Arceus with miscellaneous chip damage.

Of course any half-serious physical attacker kills you, but it's more important to talk about the special attackers that Registeel can't stop. Probably the most pressing example is Arceus-Ghost, which can harass Registeel with various special sets: the standard Knock + Spikes variant is annoying for obvious reasons, TH Ghostceus isn't stopped by Knock/Salt Registeel + Imposter, and NP Ghostceus is typically running Secret Sword, which will hurt a lot. In fact, Modest Simple Ghostceus has a 62.5% chance to kill Sassy Registeel from full with +4 Secret Sword.

Besides that, mixed mons will usually carry physical anti-steel coverage, especially V-create, which will knock out Registeel after it switches in on an initial hit. Final Gambit mons like Kyurem-W and Eternatus are also worth mentioning, as they can reliably bait in and remove Registeel to make way for additional special threats. Finally, Spikes are just really annoying if you can't remove them.