None of these people are the "rightful" heir. Westerosi kings change the rules of succession all the damn time. There's no "rightful" succession when the rules change on the current king's whim, no "rightful" when you have the rule of people but no rule of law. And none of these happy assholes have the consent of the people they govern.
These assholes are all equally illegitimate.
You do know this is a fantasy show, right ? I can root for anyone i want in a fantasy show.