Okay, here's what I'm going to say about my Zekrom's item choice - with Life Orb guaranteed 2HKO on Ferrothorn and Phys.Def Groudon at + 1 with Bolt Strike and Outrage, respectively. Groudon only has a minute chance to deal 90%+ with Earthquake in return, meaning that if no hazard damage is taken by Zekrom more often than not it will beat Groudon if it can get to +1 on the switch. This Zekrom set is a lategame sweeper first and a setup sweeper second, it only has Sub + Hone Claws to beat Ferrothorn and in reality can't run many other moves ANYWAY. Focus Blast, Hidden Power, Draco Meteor have little to no use and I would only consider Volt Switch in replacement of Sub + Hone; Draco Meteor doesn't 2HKO Groudon without investment and Sub + Hone beats Ferrothorn better than Focus Blast, so there you go.
outside of highly specialized niche counters like Sleep Talk, Choice Scarf Heracross and, to a lesser extent, Primeape, Darkrai must simply be played around. If the opposing team chooses to use a Choice Scarf Heracross then this can be taken advantage of by using Pokemon that cannot be revenged by it and thereby forcing them to use TWO scarfers or not deal with Darkrai properly.