When Generation II of Pokémon added the Steel-type, it shook up the franchise's landscape significantly. This type countered both Psychic and Dragon which, before Steel's introduction, had dominated. Many Steel-types have another typing attached to them, but some only boast the one type. There are not many pure Steel-type Pokémon and the few that exist range in strength.
One can evaluate power in the main series games by considering a Pocket Monster's stats, moves, Abilities, etc. Certain Pokémon, like Arceus and Greninja, can change to any typing through items or Abilities/Hidden Abilities. These, however, do not count as pure Steel-types. Certain Mega Evolutions, however, do qualify. If players need a creature to counter some of the more dominant types in the game, some pure Steel-type Pokémon can be helpful.
Perrserker
Perrserker, the evolution of Galarian Meowth, has the potential to be a solid offensive competitor. It has a high Attack stat and two Ability options that can contribute to this. The Tough Claws Ability increases the power of moves that make contact, and the Steely Spirit increases Hidden Ability the power of Steel-type actions. Perrserker can already learn some of Pokémon's most powerful Steel moves, and the Hidden Ability gives it an extra boost.
Unfortunately, Perrserker suffers from a poor Speed stat, and while its Defense is high, its Special Defense leaves it vulnerable. It is not so low that the Pokémon is defenseless, but opponents can catch it off-guard with a Special attack, sometimes knocking Perrserker out before it can use its great offensive power.
Klinklang
One of Klinklang's best features is its Speed stat, which is higher than that of any other pure Steel-type, though still not exceptional. It can utilize its decent Attack stat to attempt to defeat enemies before they can strike. However, since this stat is not incredible and Klinklang does not have access to many great moves through leveling up, it often does not finish battles quickly.
Its Defense allows it to take Physical strikes well, and while its Special Defense is lower, it is high enough to deal with a fair amount of damage. This is especially true in the early-game and mid-game portions. Klinklang runs into trouble as opponents get tougher, though, so there are better Steel-type options when progressing through the games.
Copperajah
Though it evolves from Cufant, one of the cutest Steel-types in Pokémon, Copperajah can be quite intimidating. An already heavy creature, its Heavy Metal Hidden Ability doubles its weight. With this, it can better utilize the move Heavy Slam, which calculates damage based on the weight difference between competitors.
Aside from Heavy Metal, Copperajah also has a great Attack stat and access to many Physical moves of different types, offering it more coverage options. Its Defense and Special Defense are lacking, but it has a high HP stat to compensate. While it may not be as strong as some other Steel-types, Copperajah is more than capable of taking and dealing a large amount of damage.
Registeel
This pure Steel-type Legendary fully embraces its typing. Registeel's Defense and Special Defense are much better than its other stats, making it a tank. It can take both categories of attacks very well since both its defensive stats have the same value. Its offensive stats also have the same value, but these are much lower.
Registeel relies on wearing opponents down over time, which it can do with a variety of attacks. Its Ability Clear Body keeps foes from lowering its stats, so its high Defense and Special Defense remain as such. Registeel is not the best Legendary in the series, but it is among the better pure Steel-types.
Mega Aggron
Aggron is a Steel/Rock Pocket Monster in its base form, but it drops the secondary Rock typing upon Mega Evolving. Aggron is already one of the strongest Pokémon from the Hoenn region, and its Mega Evolution buffs it even further. It has a base Defense stat that leaves it essentially impervious to Physical attacks.
Its Special Defense is much lower, but Mega Aggron's Ability Filter reduces damage from super-effective hits, helping to make up for this vulnerability a bit. When it has the opportunity to strike, this Pokémon can do a great deal of damage with its high Attack stat. Aggron is not fast, but with its exceptional defensive capabilities, it does not need to be.
Melmetal
A bit of an obscure Mythical Pokémon that has only recently received more attention, Melmetal is an exceptional Steel-type. It has a very high HP stat that pairs well with its high Defense, making it a wall against Physical attackers. While its Special Defense is lower, its HP more than makes up for it.
Like many other Steel-types, it is slow, but when it can strike, it devastates opposing teams. Its Ability Iron Fist powers up punching moves, which are one of Melmetal's specialties. With its terrifying power, it is no wonder why it is Ash's best Pokémon from the Alola region, which is where it debuted in the anime.