here are all the Transformations legendary ben has
Generation 1 (Kanto)Legendary: Articuno, Zapdos, Moltres, Mewtwo
Mythical: Mew
Generation 2 (Johto)Legendary: Raikou, Entei, Suicune, Lugia, Ho-Oh
Mythical: Celebi
Generation 3 (Hoenn)Legendary: Regirock, Regice, Registeel, Latias, Latios, Kyogre, Groudon, Rayquaza
Mythical: Jirachi, Deoxys
Generation 4 (Sinnoh)Legendary: Uxie, Mesprit, Azelf, Dialga, Palkia, Heatran, Regigigas, Giratina, Cresselia
Mythical: Phione, Manaphy, Darkrai, Shaymin, Arceus
Generation 5 (Unova)Legendary: Swords of Justice (Cobalion, Terrakion, Virizion, Keldeo), Forces of Nature (Tornadus, Thundurus, Landorus), Tao Trio (Reshiram, Zekrom, Kyurem)
Mythical: Victini, Meloetta, Genesect
Generation 6 (Kalos)Aura Trio: Xerneas, Yveltal, and Zygarde
Anime/Movie Featured: Diancie (Mythical but central to Diancie and the Cocoon of Destruction), Hoopa (Mythical but stars in Hoopa and the Clash of Ages), and Volcanion (Stars in Volcanion and the Mechanical Marvel).
Generation 7 (Alola)Guardian Deities: Tapu Koko, Tapu Lele, Tapu Bulu, and Tapu Fini
Cosmoem Line: Cosmog, Cosmoem, Solgaleo, Lunala
Light Trio: Necrozma
Ultra Beasts: Nihilego, Buzzwole, Pheromosa, Xurkitree, Celesteela, Kartana, Guzzlord, Poipole, Naganadel, Stakataka, and Blacephalon
Artificial: Type: Null and Silvally (introduced by Gladion in the anime)
Anime/Movie Featured: Magearna (Mythical but prominent in Volcanion and the Mechanical Marvel), Marshadow, Zeraora, and Meltan/Melmetal.
Generation 8 (Galar & Hisui)Hero Duos/Trios: Zacian, Zamazenta, and Eternatus
Legendary Giants: Regieleki and Regidrago
Calyrex Line: Calyrex, Glastrier, and Spectrier
Forces of Nature: Enamorus (introduced in Legends: Arceus)
Anime/Movie Featured: Zarude (Mythical, featured in Secrets of the Jungle).
Generation 9 (Paldea & Kitakami/Terarium)Treasures of Ruin: Wo-Chien, Chien-Pao, Ting-Lu, and Chi-YuParadox Legendaries: Koraidon and Miraidon
Loyal Three: Okidogi, Munkidori, and Fezandipiti
Masked Ogre: Ogerpon
Disk Pokémon: TerapagosAnime/Movie Featured: Pecharunt (introduced in the epilogue) and Terastapall Form Terapago
Others
Zoroark
Rotom
Greninja
Pseudo legendary
Generation 1: Dragonite (Dragon/Flying)
Generation 2: Tyranitar (Rock/Dark)
Generation 3: Salamence (Dragon/Flying) & Metagross (Steel/Psychic)
Generation 4: Garchomp (Dragon/Ground)
Generation 5: Hydreigon (Dark/Dragon)
Generation 6: Goodra (Dragon)
Generation 7: Kommo-o (Dragon/Fighting)
Generation 8: Dragapult (Dragon/Ghost)
Generation 9: Baxcalibur (Dragon/Ice)
Mega evolution
Generation I Venusaur (Mega Venusaur)Charizard (Mega Charizard X / Mega Charizard Y)Blastoise (Mega Blastoise)Beedrill (Mega Beedrill)Pidgeot (Mega Pidgeot)Alakazam (Mega Alakazam)Slowbro (Mega Slowbro)Gengar (Mega Gengar)Kangaskhan (Mega Kangaskhan)Pinsir (Mega Pinsir)Gyara / Gyarados (Mega Gyarados)Aerodactyl (Mega Aerodactyl)Mewtwo (Mega Mewtwo X / Mega Mewtwo Y)
Generation II Ampharos (Mega Ampharos)Steelix (Mega Steelix)Scizor (Mega Scizor)Heracross (Mega Heracross)Houndoom (Mega Houndoom)
Generation III Sceptile (Mega Sceptile)Blaziken (Mega Blaziken)Swampert (Mega Swampert)Sableye (Mega Sableye)Mawile (Mega Mawile)Aggron (Mega Aggron)Medicham (Mega Medicham)Manectric (Mega Manectric)Sharpedo (Mega Sharpedo)Camerupt (Mega Camerupt)Altaria (Mega Altaria)Banette (Mega Banette)Absol (Mega Absol)Glalie (Mega Glalie)
Generation IV Lopunny (Mega Lopunny)Garchomp (Mega Garchomp)Lucario (Mega Lucario)Abomasnow (Mega Abomasnow)Gallade (Mega Gallade)
Generation V Audino (Mega Audino)
Generation VI Diancie (Mega Diancie)
With Gen 7,8,9 having no legendary pokemons Guys this is basically Legendary Ben whole Roaster of Transformations It includes every Legend, Myth, Pseudo-Legend, Pokemons with Mega forms and Others like Greninja, Rotom and Zoroark(The GOAT) remind me if i have forgoten any?