Who'd be the best stallion for a Diktat mare?

none that I am aware of- but its early days yet for diktat as a broodmare sire. In terms of what stallion to use Diktat's two Group 1 winners Dream Ahead and Rajeem are both out of Cadeaux Genereux mares so consider the reverse cross.  Cadeaux Genereux is retired but maybe consider his son Bahamian Bounty or BB's son Pastoral Pursuits

Diktat has been broodmare sire to a couple of recent promising winning juveniles called Handassa and Blanche Dubawi - both by Dubawi.
While you'll need a huge budget (and likely a top mare) to get into him next year, Echo Of Light could be a cheap alternative - you are risking a lot though that his 2yo's next year are not awful!