True... I was thinking along the lines of the Martells, who believe Myrcella is the rightful heir.
If you get down to brass tacks, Daenerays is truly the rightful heir to the throne from what we know to be the truth at this point. As far as the kingdom knows, Tommen is the rightful heir since males supercede any older female siblings. For those who believed John Arryn and Ned Stark, then Stannis is the rightful heir since he did not have an legitimate children, and Cersie did not bear him any children. However, King Robert usurped the throne from the Targeryans, who were not completely wiped out, so any living Targeryan would be the heir, minus any who had given up their rights to the throne.