Question

What are all the Legendary Pokémon?

Answer 1

NOTE: This answer does not include mythical Pokemon, which are not considered legendary Pokemon in modern generations. It excludes Ultra Beasts, which are generally not considered legendary Pokemon.

Kanto
Articuno, Moltres, Zapdos, Mewtwo

Johto
Raikou, Entei, Suicune, Ho-Oh, Lugia

Hoenn
Regirock, Regice, Registeel, Latias, Latios, Kyogre, Groudon, Rayquaza

Sinnoh / Hisui
Uxie, Mesprit, Azelf, Dialga, Palkia, Heatran, Giratina, Cresselia, Regigigas, Enamorus

Unova
Cobalion, Virizion, Terrakion, Tornadus, Thundurus, Reshiram, Zekrom, Landorus, Kyurem

Kalos
Xerneas, Yveltal, Zygarde

Alola
Cosmog, Cosmoem, Solgaleo, Lunala, Necrozma, Type: Null, Silvally, Tapu Koko, Tapu Lele, Tapu Bulu, Tapu Fini

Galar
Zacian, Zamazenta, Eternatus, Calyrex, Kubfu, Urshifu, Regieleki, Regidrago, Glastrier, Spectrier

Paldea
Koraidon, Miraidon

Answer 2

You should really just look it up. It's not that hard.

KANTO
The Legendary Bird Trio
Moltres, Articuno, Zapdos.

Mew Duo
Mew and Mewtwo

JOHTO
Legendary Beasts
Entei, Raioku, Suicune

Tower Duo
Ho-Oh and Lugia

Celebi

HOENN
Legendary Golems
Regirock, Regice, Registeel

Eon Duo
Latias and Latios

Weather Trio
Kyogre, Groudon, Rayquaza

Jirachi

Deoxys

SINNOH
Lake Guardians
Azelf, Mespirit, Uxie

Creation Trio
Dialga, Palkia, Giratina

Lunar Duo
Cresselia and Darkai

Sea Guardians
Manaphy and Phione

Heatran

Regigigas

Shaymin

Arceus

UNOVA
Victini

Legendary Musketeers
Cobalion, Terrakion, Virizion, Keldo(a younger fourth member)

Kami Trio
Tornadus, Thundurus, Landorus

Tao Trio
Reshiram, Zekrom, Kyurem

Meloetta

Genesect