
Looking at Isildur’s will... he declares it an heirloom of rhe North Kingdom and not specifically his line...
“The Great Ring shall go now to be an heirloom of the North Kingdom…”
Surely it is what he meant (his heirs) but legal specifics can be a bother. It is not what the text actual says. He specifically states “the North Kingdom” as the beneficiary. Does that mean it would be in some sort of public trust? And since the north kingdom has ceased to exist for a long time... again, debatable in a court of law I think?
Now I would assume that the title would go back to the line of Isildur if the court ruled Arnor had indeed ceased to exist - if they claimed it. But they never did.
So then onto Gollum...
Now I just read that apparently there are some conditions for acquisition by prescription. One of them is the ownership should be known/notorious. Which is a problem for both Gollum, Bilbo and even Frodo. None of them owned it notoriously for long enough...
Aragorn and Frodo would be the best contenders. And since Aragorn decided not to take it.. Frodo has the best claim?