DARKRAI PWNAGE :)

No one beats him.

You CAN beat darkrai without a strategy- just use another uber :) lol, just kidding,
nobody can beat my darkrai.

But strategically speaking, I'm pretty sure that Arceus/Rayquaza can beat Darkrai.

Mewtwo... I'm not really sure since it's weak to dark types.

A lot of sub legendaries are used-to-be ubers before more powerful ones came out. For
example, Salamance used to be a uber back in earlier gens since they need more ubers- I
think the only uber in the eariler gens is Mewtwo so they added some sub-legendaries.

Legendary trios- some of them are in Ubers too like Zekrom Kyurem and Reshiram.

It does make me feel sad to see some legendaries in NU and UU though.