Zacian, the legendary Warrior pokémon is just nuts on so many levels. Pure Fairy typing is a fantastic way to start, having only two weaknesses and a myriad of resistances. 92/115/115 defenses are very durable in conjunction with the pure Fairy typing. Base 138 speed outspeeds so many pokémon that it is ridiculous. Base 130 Attack may seem comparitively unimpressive, but Zacian's ability Intrepid Sword acts essentially as a Choice Band, before the item boost, giving it a respectable degree of power.
And then comes Zacian's Crowned form. With Base 170 attack, Zacian Crowned is tied for the highest available attack in the game with Kyurem Black and completely outstrips Kyurem Black with Intrepid Sword. Fairy Steel is a contender for the best typing in the game with 9 resistances including 2 immunities and only two weaknesses. And a Base Speed of 148 ensures that Zacian Crowned has everything it needs to succeed, outspeeding all but the fastest Pokémon. While Zacian Crowned may have 60 less BST than Mega Rayquaza and 34 less than Ultra Burst Necrozma, Zacian blows both out of the water with its excellent typing, stat distribution and ability. Zacian Crowned is a force of nature unto itself and while it may be more predictable than the other two, it can and will decimate entire teams on its own if given have a chance.
World Cleaving Slash
Zacian
Ability: Intrepid Sword
Item: Choice Band
-Play Rough
-Close Combat
-Wild Charge
-Psychic Fangs
Nature: Adamant (+ATK, -SATK) or Jolly (+SPE, -SATK)
EVs: 4 HP / 252 ATK / 252 SPE
Not much to say, have Zacian hit the field, rip something apart with a Choice Banded Attack, retreat as required, rinse and repeat until the opposing team or Zacian is knocked out. Play Rough is STAB and hits many valuable targets (Rayquaza, Zekrom, Yvetal, Kyurem, Dracovish, Zygarde, Palkia, Marshadow, Zamazenta, Urshifu, etc.). Close Combat provides valuable coverage and hits many Steel types that resist Play Rough (Ferrothorn, Heatran, Excadrill). Wild Charge hits the many Flying types that Zacian will likely be butting heads with (including Ho-oh, Lugia, Yvetal, as well as Kyogre). Psychic Fangs gives Zacian an answer against Poison types like Naganadel and Eternatus.