The rightful heir to the iron throne is Jon Snow or also known as Aegon Targaryen. This is because before Robert’s Rebellion, Aerys II Targaryen was the king who ruled the Seven Kingdoms. He was killed during the rebellion and the Baratheon took over the iron throne. However, if we are to take into consideration who was supposed to rule the Seven Kingdoms if the rebellion did not take place, Rhaegar Targaryen was the heir to the iron throne because he was Aerys’ eldest son.
Rhaegar Targaryen was in love with Lyanna Stark and they secretly got married. During their forbidden love affair, Lyanna conceived a child and she named him Aegon Targaryen, but she made Eddard “Ned” Stark to keep him safe so, Ned changed Aegon’s name to Jon Snow and told everybody that he was his bastard son.