The defensive king — 11 resistances and immunity to Poison.
Steel has more defensive resistances than any other type in the game: 11 total (Normal, Grass, Ice, Flying, Psychic, Bug, Rock, Dragon, Steel, Fairy, and — depending on gen — Ghost). Combined with immunity to Poison and Sandstorm damage, Steel is the go-to typing for any wall or tank. Gen 2 introduced Steel alongside Dark as a deliberate Psychic counter, and it immediately became the most-used defensive type in the game.
Steel is a wall, tank and utility type. Ferrothorn, Corviknight and Clefable (post-Fairy typing) are three of the most common defensive cores in modern VGC. Iron Head flinch spam, Bullet Punch priority and Heavy Slam (scales with weight) are the three main Steel STAB options.
Iron Head (80 BP + 30% flinch) is the iconic physical move. Flash Cannon is the special option. Bullet Punch is priority. Heavy Slam scales with weight ratio up to 120 BP on heavy-vs-light matchups.
Aegislash's Stance Change ability — swapping between offensive Blade Form and defensive Shield Form each turn — was so dominant in Gen 6 that Game Freak specifically nerfed its base stats in Gen 8, the first time a Pokémon's stats were directly reduced between generations.