This is the order you are going to want to play the Xenoblade games.

Xenoblade X on Wii U is still very much worth playing and is awesome, but Xenoblade Chronicles/Xenoblade Chronicles 2 (and Xenoblade Chronicles 2: Torna - The Golden Country) are all related and really just one big crazy story, so those definitely belong in a set.

And even though Torna is a prequel (and the length of a full game), that is absolutely the last game you want to play in the set. The emotional impact of the game will be 10x greater if you play it after Xenoblade Chronicles 2.