How would it work?


I'm curious as to how would it work if their was A Legends Kingdom where by unbreakable tradition titles went to whichever member of the next generation of the family proved himself or herself most worthy of the title and not just the oldest surviving child of the title holder, with even Nieces and nephews having a chance at getting the title and even The Royal Family having to do this, thus insuring that every title holder was the 1 best suited for the job/title and not just the oldest surviving child of the title holder
This is not a model that ancient or medieval societies adopted as it is a recipe for perpetual political instability. When a new ruler ascends to the throne, they are not going to want surviving siblings around who have a legal basis to challenge their legitimacy through rebellion. The closest parallel in the real world is probably the rapid churn of emperors during the Third Century Crisis in the Roman world. At one point, the average lifespan of a new emperor was about 14 months. It wasn't until a badass like Diocletian gained power that things began to stabilize. And one reason that he was so tough on the young Christian church was that after 60 years of civil war he was very, very sensitive about anything that might be construed as disloyalty. So in a political setup like the one you describe, rulers would be paranoid about treason and constantly checking that no illegitimate brother or sister isn't going to suddenly pop up with a claim on the throne...
Various nobles were elected historically.

The Holy Roman Emperor was elected by a council of Electors.

I think that some of the Frankish/Germanic kings were elected, but cannot remember for sure.

In some ways, it makes a lot of sense. You don't get a weak idiot king just because he is the firstborn son, for example. However, it can mean that the potential candidates become pawns in the power plays of powerful people, which adds to the fun of a political game. It also means that a situation where there is no obvious choice can fall into civil war, again good for a game.