Due to the complexity of Galar's Pokedex, this answer mentions DLC Pokemon, but discusses them in different segments of the answer for the sake of thoroughness. They are judged against each other, instead of against Pokemon from the base game. This answer also assumes parity between Sword and Shield, which omits version exclusives from this list -- in this case, Mawile, Dialga, Solgaleo, Zacian-C, and Zamazenta-C are the only Pokemon this applies to.
In a game of hyper offense, the main criterion a Pokemon will be dubbed "strongest" by is its attacking prowess (e.g. base attacking stats, Speed, movepool, ability to hit certain Trainers). Secondary considerations are location, defenses, Abilities, evolution level, and other smaller details. It's generally a better idea to play through a Pokemon game with fast, hard-hitting Pokemon, so defenses can often be forsaken -- though they are looked upon favourably.
Base Game
The fully evolved Steel type Pokemon in the SwSh base game are Corviknight, Klinklang, Bronzong, Excadrill, Steelix, Perrserker, Ferrothorn, Stunfisk-G, Bisharp, Escavalier, Lucario, Copperajah, Durant, Aegislash, Togedemaru, and Duraludon. This list can be culled, based on the aforementioned criteria, to the top three Steel types in the region.
Corviknight
While Corviknight immediately breaks the rule of offense stated earlier, it's easily one of the best Steel types in the game. Rookidee is available as soon as the game is started, it evolves reasonably, and Corviknight has a movepool with exactly what it needs. U-turn makes it a good pivot, and Bulk Up, Iron Head, Body Press, and Brave Bird help its offensive presence. Its bulk is more balanced than Steelix and it's more offensive than Ferrothorn, leading to it being on this list instead of those two.
Excadrill
Excadrill was one of the best Pokemon in the game back in Gen 5, and it's still close to holding that title in Galar. Boasting a monstrous Attack, early availability, a good movepool, reasonable Speed, and the Ground type, Excadrill is a force to be reckoned with. It gets access to great STAB in Earthquake and Iron Head, plus coverage moves like Rock Slide, X-Scissor, and Poison Jab. With a diverse moveset and the strength to back it up, Excadrill should be a consideration for every team.
Aegislash
Aegislash has both high attack stats and high defenses, assuming you play it right. This makes it highly flexible to fit the needs of the battle, especially with a large movepool that gives it access to strong moves like King's Shield, Iron Head, Shadow Sneak, and Swords Dance. It's also reasonably easy to get a Dusk Stone, so getting this Pokemon shouldn't be too much of an issue.
Isle of Armor
The Steel Pokemon added to the Isle of Armor are Klefki, Magnezone, Scizor, and Skarmory. Due to the scarcity of additions, only two Pokemon will be detailed.
Magnezone
Magnezone is an incredible special attacker, with 130 base Special Attack and moves like Flash Cannon, Thunderbolt, and Tri Attack. Its movepool is fairly shallow, but it can blast opponents perfectly well with what it got. It's slightly lacking in Speed, but not enough to matter, and its defenses are high enough that it can usually take at least one hit.
Scizor
While Scizor is a trade evolution, it can also be found as a wanderer and in Max Raid Battles, so it is possible to obtain if playing solo. With just one weakness and the same Attack stat as Excadrill, Scizor can tear holes through most in-game opponents. Iron Head, X-Scissor, U-turn, Brick Break, and Bullet Punch all lend to its effectiveness in attacking, and its solid Defense does it quite a few defensive favours as well.
Crown Tundra
The Steel Pokemon added to the Crown Tundra are Metagross, Aggron, Registeel, Cobalion, Heatran, Celesteela, Kartana, and Stakataka.
Metagross
Metagross is ridiculously strong, and one of its only downsides is how difficult it is to catch. None of its stats are bad, its Attack and Defense are downright spectacular, and it has a great movepool, with moves like Bullet Punch, Rock Slide, Earthquake, Zen Headbutt, and Body Slam able to OHKO most opponents. Anyone wanting to catch a Metagross should expect to spend a while lobbing Poke Balls at it, though.
Kartana
The Ultra Beast quests in Dynamax Adventures take quite a while to get to, and it's not a guarantee that Kartana even appears within a reasonable amount of encounters. It takes significant RNG to get this Pokemon, but it's very powerful if accomplished. Boasting 181 Attack and 109 Speed, Kartana will OHKO most opponents easily, especially with strong moves like Leaf Blade and Sacred Sword. It's so tedious to get to, though, that it might be more effective to simply beat the game with a different Steel type.
Note: Some consider Ultra Beasts legendaries, and others don't. For the sake of thoroughness, Kartana has been included, but feel free to ignore it if you consider it a legendary.
In conclusion, the best Steel types are those that are fast, can deal a lot of damage, and have a wide movepool. Some Pokemon in this list are more effective than others, but because this is a playthrough, practically anything will be effective in beating the game.
