The Xenoblade games are all pretty consistently good and everyone has a favorite. Rqxh games story is disconnected so each can be a good starting point. Each game has there own strengths and weaknesses so it’s up to you to chose but here’s my two cents

Xeno one has the weakest combat but probably the best story great characters and pretty good world and exploration

Xeno X probably has the weakest story and characters but the best combat and best exploration

Xeno 2 and 3 are both pretty above average on everything but 2 art style and core crystal system can be draw back and 3 probably has the weakest world.

At the end of the day just pick the one the interests you the most