So as I wrote in the topic section, XCX is my favorite of the Xeno series, by far and I played the entirety of Xenogears recently (as well as when it first released) as well as through all 3 Xenosaga's although I can't say finished them as of yet, I had seen the anime, I have played through most of it, and I do know how it ends. I have beat Xenoblade Chronicles and X, and have gotten half of the way through Xenoblade 2. This is just to preface what I have played and where my perspective is coming from.
That said Xenoblade X to me stands head and shoulders above the rest. The complete open world is simply the best designed map of any Xeno in my opinion due to the many aspects of clever design. There are some places that can only be reached by foot, some only by flight, and there are numerous hidden rooms throughout each of the environments hiding rare weapons, secret bosses, etc. There are also no invisible walls, or edges of the world, and your character is capable of jumping exceptionally well (and there is reason for this) so with a few rare exceptions you glance at where the arrow is for the next mission and go. I found this SOOOO frustrating in Xenoblade 2 where I pretty much had to keep my map on full size, missing all the beautiful world to be able to make sure I take the right path to stay on mission.
Second the skells are great because you can equip essentially any skell to any character, and any skell (lvl60 and up mind you) can be customized to defeat the the hardest bosses. You can have a 60 Cain that is better than the Ares 70 or 90. it's just awesome to know you can change things up without any handicap really. Love this as it is not required to beat the game or any monster, you can see people on foot take out the hardest enemies such as the telethia and rather easily I might add. It also throws out a huge problem I had in the Xenosaga episode 1, in that you feel like you lose a turn every time you hop in and out of your AGWS. This game you can jump in and out of your skell as you see fit with no handicap.
Third is the music. I know it has gotten a bad rap for some of it's music, but holy crap Theme X, Your Voice, The Way, and Uncontrollable are some of the greatest music to hit jrpgs ever, and this is just off the top of my head, there is the noctilum night song and numerous others that just blow you away.... Agree that this game has some of the worst music in the series, but it easily has the top 3 to 5 tracks of the entire series as well with other songs going down the list accordingly.
Fourth is the in game economy. There are several different factors at play in how the world works in Xenoblade and they all do so brilliantly. You have the gamepad functionality with the mining and fortune probes, and these are important because you need money for research in arms tech, and miranium for skell fuel and crafting. You have the online squad missions which you don't have to actively do at all but gets you reward tickets when any of the monsters are slain that are tasked to be, and this is used for buying rare materials for crafting. The actual online mode is spread out to make everything easier as far as you can farm reward tickets, gain lots of affinity for characters, gain a lot of money, or miranium, etc. you can do it all. Nothing feels tacked on and everything is useful. You'll here a ton of people say the online is tacked on, but I disagree. It all serves an important purpose if you know how to utilize it.
Fifth is the gamepad. You want a full map without having it be displayed on the tv, done. Dealing with probes and seeing details on specific areas with the map with the convenience of a touch screen, done. It streamlines all the economic systems, and clears the clutter from the television, what could be better than that? Some people will say "is that it", and I would simply ask what more do you need? The gamepad isn't over utilized to the games detriment (see Star Fox Zero) it simply is there as a companion piece, and that is perfect.
Sixth is the characters. Play the game and tell me Irina and Lao didn't have you with sand in your eyes. Elma is awesome, loved helping Mia along with becoming a Blade member and all the trouble that came with that, lol. There are a LOT of great and memorable characters in this game. with that though (due to the cast being fairly big for a Xeno game) some are rather static. Overall though I loved the cast of this game, and doing all their affinity quests gains you new ultimate arts for a class. It makes you want to know these characters and they get fleshed out and you grow more attached because of it. Just saying if they kill off Irena in a sequel, it would devastate me. Just a ton of rewards in this game, and completing the affinity quests is very rewarding on multiple levels.
Last thoughts about this is that people will say the story is bad, and to a degree I will agree with them. It's a first game in a series (hopefully) and it plays out as much. The game is based entirely around an immediate threat, while having a lot of bigger picture stuff going on which doesn't get any traction in this game. So here's hoping for a sequel or two to get to the bigger picture stuff moving forward. There is good lore, good gameplay mechanics, a breathtaking world and map. It would be a terrible tragedy if this timeline of the game were not able to continue.