I'm going to disagree on story. I found Xenoblade Chronicles 1 and 2 to be fairly unremarkable and unoriginal in the plot department. They were rehashes of much more interesting and thorough Takahashi/Soraya Saga stories (both games are like vertical slices of the vastly superior Xenogears).
I think, in the end, the reason I like XCX's story more is the scope of the story. XC1 and XC2 are not only much more linear, but they're a lot smaller in scale. Basically each one is a heroes journey story (which are like a dime a dozen in RPGs). XCX is significantly original, it puts the player in the center of a larger story, an entire city and its struggle against an alien planet - as the player, you go to various parts of the city and help them with the struggle, and it's all part of the whole.
As well, Xenoblade Chronicles X had a better world than XC1 and a significantly better world than XC2 (which wasn't terrible, it was actually better than most RPGs, but fell well below the bar set by the other two Xenoblade games).
My main issue with Xenoblade Chronicles 1 and 2 are that they felt a little too on-the-rails in both map and story design. The outer maps of XC2 lack the spectacle of XC1 and XCX, although the cities are nice. XC1 has some great areas (Fallen Arm, Makna Forest, and Frontier Village) but it's a mixed bag with some very bland and sterile areas (Sword Valley, Mechnonis interior, Bionis interior, the snowplains/mountain maze place, Prison Isle, Eryth Ocean, and the High Entia Tomb, the colony 6 mines) which, unfortunately, take up a significantly larger portion of the game. The worst area in a Xeno-game still remains the Encephalon from Xenosaga, but some of the locations of XC1 come painfully close.
Xenoblade Chronicles 2 is kind of like 2 games. The first 6 or 7 chapters, and then the last 3 or 4 chapters. The last 3 or 4 chapters are basically one large dungeon that's reminiscent of The Tower of Babel from Xenogears, and a little of Zeboim civilization from Xenogears. While the World Tree (or whatever it was) is more fun than the Tower of Babel (particularly the lower portion with the annoying platforming), the ruins of Zeboim Civilization are SIGNIFICANTLY more interesting than Morytha, though. None of the games have a "dungeon" type area as interesting as Solaris, which IMO is (to this day) the best "dungeon" in an RPG ever (I put dungeon in quotes, because it's not really a dungeon, it's more like a country inside of an Archology).
IMO Torna the Golden Country was better than the first half of XC2, but not as good as the second half. New Game+ of XC2 is the most fun experience I had with the game.
Anyway, XCX is a much larger game than the other two Xenoblades. But DESPITE being larger, XCX was fairly remarkable in how much more manageable the play sessions were. Since it was non-linear, players were presented with options for short, medium, long, and really long bits of the game they could do. Then there was also a lot of random curiosity stuff, exploring some new area with your new Skell/Gear. The non-linearity is refreshing, XC1 and 2 don't really do anything better, storywise, they just shove more into the critical path; and for the most part, you're forced into the same experience, while the experiences you can have with XCX are vastly different depending on which paths you take.
The big difference with XC1 and 2 vs XCX is in XC1/2 the story is spoonfed to you as you go along the rails. In XCX you seek out your own way to move through the story of a city from earth and its struggle against the alien planet it has been plunked down upon.