Definitely X and Y. Don't get me wrong, I love Mega Evolution, but I just don't like it all that much. The rivals weren't all that memorable, and I bet the only reason anyone WOULD remember them is if X/Y was their first game or they like the anime. The evil team was basically Team Galactic with elements of Team Rocket and Team Plasma; Lysandre wanted to use a legendary Pokémon to make a better world like Cyrus, the grunts believed their only goals were to gather Pokémon and money and stuff along those lines like Team Rocket, and the team had a mad scientist like how Ghetsis has Colress (I would normally see all this as a dream team, but it's a hot mess). And to top it all off, the only reason anyone remembers the post game is because of Looker being the driving force behind it. Replace Looker with someone else, I guarantee that only fans recently brought in BECAUSE of X/Y would remember it.
Red/Blue/Green/Yellow were the first games, so they had an excuse, as well as a memorable rival. Gold/Silver/Crystal/HGSS had a whole region and other boss to explore, as well as a memorable rival. R/S/E had a LOT of legendary Pokémon, the Battle Tower/Frontier, AND I still enjoyed mindlessly re-battling the Elite 4 and Wally, even if the main rival was lackluster. D/P/Pt had a whole island, with Platinum adding another final Team Galactic showdown, AND a memorable rival. B/W had a whole second half of the region to explore and TWO memorable rivals, as well as PokeStar Studios and the PWT in B2/W2, despite those last two having a lackluster rival. And of course, as every current fan that stays up to date knows, ORAS had the Delta Episodes, which I found thoroughly enjoyable, AND they made the lackluster rivals from the original R/S/E much more memorable. X/Y, though... I don't see ANY of it.