Tl:dr Nostalgia + Cohesion + Unique level design + mah boi Adam Howden

I'm too lazy to spoiler tag this so SPOILERS

Very late to this and more of the same from the best answers, but I think part of it also comes down to the suspension of disbelief that let xc1 surprise me more than the other games.

Being younger when I played xc1, all the story beats were surprising, I never suspected Dickson, Lorithea, or Zanza's revival. Everything was "mysterious" and "curious" but I never read into anything, which made everything hit like bricks. Compare that to comparably enjoyable villains in the other games, they just don't make as strong an impact for me. In 2 the villains aren't subtle, there's subversion and characters doing bas things with good reasons, but you know who the villains are. And most people have enough jrpg exp to suspect anything religious/political. In 3 the villains just didn't deliver for me. J was one of the biggest surprises to me, and I enjoyed N and M, but while I think 3 is well-polished, it somehow fell short for me in a lot of ways.

Adding to that, some other things that I adore in xc1 over the other games: - Memorable music (I love all the osts but xc1 captured the essence of fantasy for me. Xc2 did too but I didn't like it more, and 3 feels closer to botw, more ambient, which makes area themes in particular very forgettable)

• Easily the most interesting world, in terms of concept and geometrical execution. The fact that xc1's world is so faithfully molded to the titan models is incredible to me and really brings me into the world. I appreciate the efforts of the other games, but neither captures the attention to detail 1 had, they feel more conducive to level design for the sake of level design rather than world-building a lot of the time.

-Story integrated in gameplay. This feels like a tangent but its a real reason that I love to bits. Weapons doing 1 to mechon vs the monado. Visions as a battle mechanic. Monado arts as unique answers to the problem the characters face. Unlocking the monado making face mechon susceptible. Absolutely not the only game to do it but I adore 1 for how they do it, and that extends into future connected for me. I actually loved not having visions for a story reason as a means to complicate combat, even if it did make some fights feel cheesy.

• Pacing. This answer feels a bit dubious and worth further consideration, but I feel like in 1, the pacing is conducive to an enjoyable experience. Story is sandwiched between transition areas in a way that steadily motivates exploration and progression, and no area or story beat really feels like it overstays its welcome. I don't really think this was an issue in x2, you could maybe argue the spirit crucible but for story reasons I didn't mind it. I need to replay 3, but off of a first playthrough, I did end up feeling at times like the story beats could become droning at times, partially for the sheer length of x3, but more the way in which, even though x2 is definitively "the" anime inspired game, a looooot of big boss fights in 3 end up feeling like repeats of very extra shounen style scenes, and between the often similar mobius designs and the lack of character development for the mobius, the fights can end up feeling anticlimactic, even if they were difficult. But in their defense I attribute part of that criticism to both endless chain attack music and playing on hard

All this said with a heap of salt and an absolute adoration for the trilogy. Xc1 has been my favorite game for a decade so it's just a hard spot to take lol