Sun and Moon. It took everything X/Y had and made it 1000x better. And the story actually had thought put into it for once. X/Ys story was laughably bad. The region was nice but it had a lot of wasted potential.
However, despite me preferring S/M more, X/Y do 3 things better; online, not nearly as much lag, and WAYYY better clothing variety. To me it seems like S/M wasn't initially going to have clothing at all, but was tacked on once they realized they can't go back to standard trainer designs anymore (ala ORAS)