There is a list of Legendary Pokemon catchable in Dynamax Adventures but there isn't a particular list about which is the easiest and which is the hardest. I'll mention the legendaries so you and anyone who sees the question gets the available legends in the game.
Articuno
Zapdos
Moltres
Mewtwo
Raikou
Entei
Suicune
Lugia (Shield only)
Ho-Oh (Sword only)
Groudon (Sword only)
Kyogre (Shield only)
Rayquaza
Latios (Sword only)
Latias (Shield only)
Dialga (Sword only)
Palkia (Shield only)
Mesprit
Azelf
Uxie
Heatran
Giratina
Cresselia
Reshiram (Sword only)
Zekrom (Shield only)
Kyurem
Tornadus (Sword only)
Thundurus (Shield only)
Almost there. (Breaths)
Xerneas (Sword only)
Yveltal (Shield only)
Zygarde
Solgaleo (Sword only)
Lunala (Shield only)
Necrozma
Tapu Koko
Tapu Bulu
Tapu Fini
Tapu Lele
Nihilego
Buzzwole
Pheromosa
Xurkitree
Celesteela
Kartana
Guzzlord
Necrozma
Stakataka
Blacephalon
Here's the source I used to answer this question.