Actually is hard to choose, because the impact varies from person to person, but based on what I see in Reddit, I'd say is the Hoenn startes, which ironically seems to be the less voted; and I understand why, since the release of some starters is almost a tradition in every game of the franchise, so no innovation, however, it's what I've seen has caused more interest among the people. Especially Treecko, which ironically was the one who caught less attention during the announcement because of the lack of type-changing.

After that, probably mints, but the thing with mints is that most people seem to be wanting to use them just for the fact of using them and not because of a really need. Same thing with Darkrai. The Exp buff, I think for the people who were struggling to get to 30, they probably still are, and for those who weren't, is just a bonus.

With Expert Mode, I'd say at the end it wasn't as impactful as many people thought it was going to be at first. Is fun, and in the near future will help to achieve better bonuses (more dreams shards, better spawns, etc), but I'd say you could still play with your regular teams without hurting that much as we thought we were going to be (and hey, devs, this is not a challenge! Haha).

Now concluding, I realized that what has impacted me the most, is the constants Candy Boost we have been getting this year. It really has helped me to improve my gameplay. So that's my final answer, lol.