Voting Ho-oh. It defined XY and ORAS more than any of the other options. Darkrai wasn't as big a threat in XY when MMY and MGar were the 2 most popular megas, scarfers were popular and good, and there were a multitude of viable sleep absorbers. Salamence is a bigger threat in ORAS but Ho-oh consistently shaped teambuilding for an entire generation and undeniably had more impact.
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.
Edgar and ZF made good points, so I'm changing my vote to Ho-oh.
One thing worth mentioning is that this isn't which mons were necessarily best (from a viability rankings perspective) but rather most influential. The amount of support Ho-oh influenced is a point in it's favour.
Its defined a playstyle for both parts of the generation and forced specific things out of teams to deal with it and make the most of it, whereas Darkrai can be checked by pokemon that would likely fit teams regardless of if Darkrai existed, and that applies(d) to both sides of gen 6. Darkrai itself also fits less teams - even offensive ones. I feel that Darkrai's lower (relative to oras) threat level in xy helps brings it down a rank below Ho-Oh in this ranking when it comes to gen 6 overall, even if it is a ridiculously scary offensive threat.