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Great League is unironically the most balanced League by a considerable amount.
Giratina is so OP in UL that he's warped the entire meta around him, all that exists above rank 8 is Giratina, Giratina counters, and Giratina counter counters.
ML is just your standard ubermon core, with the added bonus that spoofers will always have a better ubermon than you unless you luck out and get 3 100% ubermons that work well together.
Great League has a meta core (Azu/Regi/Skarm/Altaria/The Mudfish), but they're diverse enough that you can make an argument for almost any Pokemon.