Hard to tell since I don't troll BG enough to see if they came up with the base dmg and tier multiplier yet... assuming it works like blm nukes... and I don't have it yet myself to test...
But I'd imagine cosmos would be the best then probably ma of Hell's Guardian/Atma of the Ultimate
IIRC, the only atma which is specific to darkness-element spells is Atma of the Cosmos from Iratham, so if you're looking to specifically boost Impact's damage and not worrying about other spells, drop Atma of the Beyond (which is what most nukers are using these days to power up ice spells) and use Atma of the Cosmos instead.