Darkrai

Great League: N/A

Currently Darkrai is not eligible in Great League due to its high CP, though this could change if it would be obtainable in special research in the future. 

Ultra League: 1 / 5

Darkrai suffers in Ultra League due to a high ATK stat which limits its overall bulk due to the CP cap. This is significant enough that it causes Darkrai to lose to certain opponents which have disadvantageous typings against Darkrai, such as Cresselia. While Darkrai does have a good moveset and its typing is favorable against the meta-centric Giratina, the lack of bulk prevents Darkrai from performing well against the overall meta. 

Master League: 3 / 5

Darkrai possesses one of the highest ATK stats in the game, only being outshined by Mewtwo and Landorus-T as far as relevant threats go. Being a Dark type also allows it to threaten the numerous Psychic and Ghost-type Pokemon in the league. Unfortunately, that is probably the extent of Darkrai's standout traits. Darkrai is fragile and has a difficult time making use of its charge move focused, somewhat expensive moveset, making it an inconsistent threat. Perhaps Darkrai's biggest issue is that Yveltal outshines it in almost every way. Yveltal possesses roughly the same moveset, but has far greater bulk to increase its consistency, and still packs plenty of power despite having less ATK.