>>716513719 (OP)XCX is relatively standalone with only minor connections to the rest of the series. It has fun gameplay and best in class open world/exploration, but the story is pretty weak. Go in expecting not much better than a typical Bethesda game story, and you will be fine.
XC1, XC2, and XC3 all have excellent stories, but are structured less open world, and more like larger zones. 1 has a more classic JRPG feel in it's setting and tone, while XC2 feels more like a shounen anime, and XC3 feels more like a 2010's seinen. Gameplay wise, 1 is also the most straightforward, 2 adds a lot of complicated systems that enhance certain parts of the game (combat is amazing here) but makes other parts of the game feel like a waste of time (Merc Missions, Field Skills). 3 takes what worked best in 1 and 2 and mixes it together while trimming the fat and either getting rid of any element that was subpar, or streamlined it to be point of being inoffensive.
Narrative 1, 2, or 3 can be played in any order. 3 does follow and reference the events of 1 and 2, but knowing 1 and 2 isn't necessary to understanding 3. The same cannot be said of the DLC expansion for 3 called "Future Redeemed" however. That needs to be played last and will make 0 sense if not played last. Future Redeemed also spoils several events and reveals from all other 3 mainline games.
I'd say play X, and if you like it's gameplay but want more like it (but not exactly the same as it) but with a much better story at the cost of more limited freedom/exploration, pick up any of the mainline series that interests you most after. Just don't touch Future Redeemed until you have done everything else.