Of course, Jon is the rightful heir to the Iron Throne by any standards.

No he isn't. Targaryen's no longer own the Iron Throne, an heir means successor after the current ruler. His is a claim, a strong one that is.

Robert Baratheons claim to the throne was based on his grandmother, Rhaelle Targaryen, the daughter of King Aegon V Targaryen.

And he used that claim to get the Iron Throne, now it is his family who is the rightful heir.

Rightful heir is Robert's children or close relatives, or distant relatives that are Baratheon. Not another dynasty.

And there, by any standards, Jon is the rightful heir, since he has a stronger claim than Dany and they are the only Targaryans left.

No, he just has a strong claim. Not inheritance of Iron Throne after current ruler on it, which currently is Cersei. Which also makes things a little more complicated... since Iron Throne at the moment belongs to Lannisters, which Cersei claimed as mother to last 2 kings and wife to last King before them.