Fairies, especially Clefable (who can set up on you). Resist Dark Pulse and hit you back super effectively.
Fair enough, though if Clefable comes in, would Sableye really stay in himself? Base Sableye is slower, so even with Taunt, the Mega grants it at least one CM without Prankster. If it's Unaware, it doesn't even need to boost itself.
(Calcs made with previous stat spread)
4 SpA Unaware Clefable Moonblast vs. 252 HP / 0 SpD Sableye: 224-266 (73.6 - 87.5%) -- guaranteed 2HKO
4 SpA Sableye Dark Pulse vs. 252 HP / 0 SpD Unaware Clefable: 49-58 (12.4 - 14.7%) -- possibly the worst move ever
Magic Guard (Assuming switched in on Sableye's first boost)
4 SpA Clefable Moonblast vs. +1 252 HP / 0 SpD Sableye: 152-180 (50 - 59.2%) -- guaranteed 2HKO
4 SpA Clefable Moonblast vs. +2 252 HP / 0 SpD Sableye: 114-134 (37.5 - 44%) -- guaranteed 3HKO
+1 4 SpA Sableye Dark Pulse vs. 252 HP / 0 SpD Clefable: 74-87 (18.7 - 22%) -- possible 6HKO after Leftovers recovery
Without Leftovers and/or Prankster, Sableye will never survive 2 hits from even unbolted Clefable, and even if it gets a second boost in, it would have no time to do anything with it.
I certainly understand what you mean about Taunt's utility to Mega Sableye, but Clefable might be a poor example, since it'd counter Mega Sableye even without boosting alongside it.