For the purposes of this answer, Mythicals are considered Legendary Pokemon. Events' availability may differ by region and game; check respective pages for details.
Gen I
- Mew
Gen II
- Mew
- Celebi
Gen III
- Articuno
- Zapdos
- Moltres
- Mew
- Raikou
- Entei
- Suicune
- Lugia
- Ho-Oh
- Celebi
- Latias
- Latios
- Jirachi
- Deoxys
Gen IV
- Mew
- Raikou
- Entei
- Suicune
- Celebi
- Jirachi
- Deoxys
- Regigigas
- Manaphy
- Darkrai
- Shaymin
- Arceus
Gen V
- Mewtwo
- Jirachi
- Deoxys
- Dialga
- Palkia
- Giratina
- Darkrai
- Shaymin
- Arceus
- Victini
- Reshiram
- Zekrom
- Keldeo
- Meloetta
- Genesect
Gen VI
- Mewtwo
- Mew
- Ho-Oh
- Celebi
- Kyogre
- Groudon
- Rayquaza
- Jirachi
- Dialga
- Palkia
- Giratina
- Manaphy
- Darkrai
- Shaymin
- Arceus
- Victini
- Kyurem
- Keldeo
- Meloetta
- Genesect
- Xerneas
- Yveltal
- Zygarde
- Diancie
- Hoopa
- Volcanion
Gen VII
- Mewtwo
- Mew
- Raikou
- Entei
- Lugia
- Ho-Oh
- Celebi
- Latias
- Latios
- Kyogre
- Groudon
- Jirachi
- Dialga
- Palkia
- Manaphy
- Darkrai
- Shaymin
- Arceus
- Victini
- Tornadus
- Thundurus
- Reshiram
- Zekrom
- Meloetta
- Xerneas
- Yveltal
- Zygarde
- Hoopa
- Silvally
- Tapu Koko
- Tapu Lele
- Tapu Bulu
- Tapu Fini
- Solgaleo
- Lunala
- Magearna
- Marshadow
- Zeraora
- Meltan
Gen VIII
- Articuno-Galar
- Zapdos-Galar
- Moltres-Galar
- Mew
- Celebi
- Latias
- Jirachi
- Dialga
- Palkia
- Manaphy
- Darkrai
- Shaymin
- Victini
- Genesect
- Volcanion
- Magearna
- Marshadow
- Zeraora
- Melmetal
- Zacian
- Zamazenta
- Eternatus
- Zarude
Gen IX
- Mewtwo
- Mew
- Deoxys
- Dialga
- Palkia
- Darkrai
- Keldeo
- Zarude
- Pecharunt
List of event Pokemon distributions (Bulbapedia)