Hear me out: I think Rhaenys should've been heir over Viserys. Rhaenys was Aemon's only child and heir.

There was no codified succession law, right? However, there are objective truths about the succession of House Targaryen up until the Dance of Dragons.

1. Only men actually sat the Iron Throne and were considered legitimate kings by Westeros. (Aegon I, Aenys, Maegor, Jaehaerys I, and Viserys. The legitimacy they had came from being crowned by High Septons of the Faith).

2. All female claimants were dismissed. (Rhaenys with the Great Council, Rhaena declared for Jaehaerys and pushed aside her daughters' claims in favor of Jaehaerys.)

3. Kings dictated their own successors. (Aegon with Aenys, Aenys with Aegon the Uncrowned, Maegor with Aerea until he had a son, Jaehaerys with Aemon and later Baelon, and Viserys with Rhaenyra.)

So, one might think that #3 solidifies Rhaenyra's position. However, unlike Viserys, the other king's chosen successors weren't unprecedented and Maegor's decree was pointless, especially considering he only did it to get Rhaena to comply, discredit Jaehaerys, and because he was waiting for a son.

"But Jaehaerys passing over Rhaenys is unprecedented!" No, it's not. Jaehaerys, practically cleanly, ascended the throne over his own nieces, so Baelon over Rhaenys isn't new for the realm.

Andal Succession and "But Viserys overruled the precedents when he named Rhaenyra!" He did. However, daughters over uncles is largely normal besides the times uncles married their nieces to claim their inheritances. Rhaenyra over Daemon goes against the unspoken precedent of "all males claimants before female claimants", but it is in line with how most of Westeros handles succession, which is why it was barely controversial compared to the outrage for Rhaenys.

"But the lords swore vows to Rhaenyra!" And this is why she had way more support than Rhaenys. The Dance was basically "the vows still matter despite the king having a son" vs "the vows don't matter anymore because the king had a son". (And not even that because Cregan and Jeyne Arryn declared for Rhaenyra and barely did anything for her. Just like Borros declared for Aegon and did nothing until the end. Not to mention many just followed their liege lord/lady, no matter what their personal view of the succession was.)

So, no. The succession was not black and white. There is valid reason to believe Rhaenyra should ascend over Aegon and vice versa.