The first games that were available worldwide simultaneously. At least until someone leaked.
On paper, the first 3DS generation should have been notable for being the first games in the franchise to adopt 3D exploration, but it didn’t really turn out that way. The two big items that generation instead brought was Mega Evolution, and the main games of the generation being hilariously easy (no gym leader had more than three Pokémon, only two other trainers in the game use Mega Evolution against you, you are guaranteed to get the best gift Pokémon ever in the Mega-capable Lucario).
X and Y do have a lot of things that make it worth recommending, though. Although the option to pick a female character had existed for nearly 15 years by X and Y’s launch, the introduction of full character customization unlocked new levels of expression for the player. (Especially if you picked “Serena,” since the female player character had an absurd number of additional options over the male one.) An underrated aspect of the game is the “PSS” system for online connectivity; although not really useful anymore unless you’re at a StreetPass reunion, it would instantly identify online players and allow for quick selection of trading or battling if so desired. Thankfully, Omega Ruby and Alpha Sapphire kept the system intact, because why mess with a good thing?
The introduction of the Fairy type was a much needed one, and not just because of the ability to null Dragon damage entirely. It also gave a much needed buff to a couple of attacking types in Poison and Steel which had been largely impotent before, with the former only strong against one type that would frequently get paired with Poison for neutrality and the latter that was offensively redundant given that the Fighting type had the same effectiveness and added three types on top of it.
Another point of criticism came when there were only 72 new Pokémon species added, and with three “mythical”/event Pokémon that left a total of 69. But in reality, the Mega Evolution mechanic added an additional 50 designs by the time Omega Ruby and Alpha Sapphire came around, a lot of which were extremely cool. The stat balancing could have been done better - I don’t think Tyranitar, Garchomp, Salamence, and Metagross needed Megas, but at least Dragonite will join the party in Z-A. And the average new species is going to be at least usable with a really good design.
Omega Ruby and Alpha Sapphire aren’t really seen as the definitive Hoenn region experience - that’s still Emerald in most people’s eyes - but I think they’re probably the most underrated duo in the franchise. For one thing, they drastically reduced the number of Surfing that has to be done in order to reach the endgame (ironic that this is where the “7.8, too much water” meme was spawned from) with the Soaring mechanic. The 3DS remixes the trumpets of the GBA extremely well, and I love it when I can get access to the “National Pokédex” before the post-game because it dramatically increases options for team building - and I think the Delta Episode is the coolest postgame story in franchise history. YOU RIDE A LEGENDARY POKEMON INTO SPACE. How cool is that?
I think I’ve replayed X version more than any other Pokémon game ever that didn’t occasionally throw a “The file data is destroyed!” message. The sheer variety of Pokémon that were available in the average 3DS game made exploring routes exciting, and I actually did monotype playthroughs with every single type a decade before combining that and Nuzlockes became a Pokémon YouTube staple. Maybe I’ll do that again with Z-A if they give Lumiose City enough spots where Ice types can hang out?