(Spoiler Main) Most worthy Targaryen to never become Ruler
Wich Targaryen would you say were the most capable, but never became King/Queen. Either through premature death or because of the succession, etc.
My top 3 list: 1. Baelor ,,Breakspear" Targaryen Its literally stated that he was an awesome guy and heir. His death is counted as a true tragedy for the realm.
Maester Aemon A very humble guy, who was actually the fabourite to inherit from Maekar, before Egg. He has the patience and disposition of a wise ruler
Aemon/Baelor (sons of Jaeherys) I pick them both as one, because both seem capable and their deaths are kinda treated as one big tragedy. Either one of them living could have prevented the Dance. If Aemon lives there is no succession crisis and maybe he even has more children than just Rhaenys. If Baelor lives Viserys has enough time to make more children before he becomes King and doesnt rush Aemma so much, in order to get Kids.
Honorable Mentions:
Daemon Blackfyre - technically not a Targaryen, but he kinda is since, both parents are Targs and he was legitimized. He seems to have been a good guy and capable. He probably wouldnt have rebelled either, if people didnt get in his head.
Jon Snow - assuming R+L=J is true of course. I doubt he will become King, but he has the Temperament and disposition of a King. He was raised with honorable values and didnt grow up in the absolute luxury of a trueborn Royal child. Since it hasnt happened yet he is only a honorable mention
Daenerys - same thing as with Jon. Actually she and Jon would be a good King/Queen combo, but Westeros isnt that lucky....