Partner: Anna the Azelf <3
Even without Dark Void like Miltank said, I still believe the Darkrai would be the strongest, because of the fact that he can still put your Pokemon to sleep with Hypnosis (unless that's also banned haha) and sucks the life out of them with his ability and the moves Dream Eater and Nightmare.
I don't do competitive battling that much anyways xD
Hypnosis/Grasswhistle/Sing pretty much has a 50/50 chance of hitting with its 55% accuracy compare to Dark Void's 80%. Which is why Darkrai's signature Dark Void is just mighty uber. Not to mention Dark Void hits all the Pokemon in Triple Battle too. (Not sure about the other sleeping moves lol)
I'll add a few myself, mostly comic with Mewtwo & Mew