Below is a list of all types--up to date as of gen 9--showing the non-legendary Pokémon of that type with the highest BST, excluding more powerful forms, such as megas (regional forms, or different equally powerful forms, such as the Aegislash forms, are okay):
Note: this answer assumes Mythical and Sub-Legendary Pokémon count as Legendary Pokémon, but Ultra Beast and Paradox Pokémon do not.
Bug: Tied between Slither Wing, Pheromosa, and Buzzwole at 570
Dark: Tied Between Hydreigon and Tyranitar at 600
Dragon: Tied between Archaludon, Baxcalibur, Dragapult, Kommo-o, Goodra (both regular and Hisuian), Hydriegon, Garchomp, Salamence, and Dragonite at 600.
Electric: Raging Bolt at 590
Fairy: Iron Valiant at 590
Fighting: Kommo-o at 600
Fire Gouging Fire at 590
Flying: Tied between Salamence and Dragonite at 600
Grass: Iron Leaves at 590
Ground: Tied between Great Tusk, Sandy Shocks, and Iron Treads at 570
Ice: Baxcalibur at 600
Normal: Slaking at 670
Poison: Tied between Nihilego and Iron Moth at 570
Psychic: Metagross at 600
Rock: Tyranitar at 600
Steel: Tied between Metagross, Hisuian Goodra, and Archaludon at 600
Water: Hero Form Palafin at 650
Source: Pokédex with stats--Pokemon Database
Hope I helped!