Darkrai being the best goes without saying. Most of the others are unplayably bad. I mean, the next best is Jirachi, and even that's barely usable. (If Celebi didn't have that coinflip, it would probably be better than Jirachi, though still not great.)
That being said, Darkrai isn't THAT good, either. Though the two-flip sleep is nice, and has been criminally underused. (Why is Poison the only Special Condition that gets buffed often?) 80 for 3 is about as mediocre as you can get for an attack worth considering. If Yveltal didn't exist, I could have seen this maybe taking the role of non-EX Darkness attacker.
I'd expect Arceus and Genesect to be better than Darkrai. Possibly Keldeo and Meloetta, too.