Darkrai.
Ho-Oh is definitely more versatile but Darkrai alone is better. Its threat level is so high that most ORAS teams are forced to carry either Klefki or Scarf Xerneas. By virtue, the Pokémon forces at least one sleep-fodder, threatens to boost and wall-break/sweep, and provides certain defensive utilities as well despite its frailty thanks to its typing, such as being able to switch into Yveltal or Arceus-Ghost.
Undoubtedly, both Darkrai and Ho-Oh affect the nature of team-building in the metagame but options are more scarce for Darkrai since Darkrai itself requires much less support in working well given the correct plays.