It is a common belief (or at least that is my impression) that Aegon VI Targaryen / Young Griff will not be a good ruler because he lacks experience, while Daenerys will be a good ruler because she will have experienced Mereen. But that is a far more complex question, and it might be possible to examine it by looking at one of best Hungarian-Croatian kings in history – Matthias Corvinus.
Matthias’ situation was – whether George Martin is aware of it or not – extremely similar to that of Aegon in the books. Matthias Corvinus was son of a capable soldier and administrator, John Hunyadi. Born in 1443., he spent very little time with his father. Instead, he was raised by his mother, who managed his education. Matthias’ primary tutor was one of the most learned men of Europe of the time: Gregory of Sanok. Because of this, Matthias would become an enthusiastic supporter of education and science. As a child, Matthias learned many languages and read classical literature, especially military treaties. He spoke Hungarian, Italian, Latin, Polish, Czech, German and likely Croatian and Romanian.
From early on he had very capable mentors and allies. As a child, he had been engaged to Elizabeth of Celje, granddaughter of Despot of Serbia, Đorđe Branković. While Matthias had been imprisoned in Buda on 14 March 1457. alongside his brother, who was executed, his mother and her brother Michael Szilagy staged a rebellion against the king. After death of king Ladislaus, Michael Szilagy used threat of force to ensure Matthias’ election as a king. Between that, support of small nobility, support of papal legate Juan Carvajal and the fact that it was the only way to avoid civil war, Matthias Hunyadi was elected a king. In course of that he – or rather Michael Szilagy – made an enemy of palatine Ladislav Gorjanski who realized that his daughter Ana will not become a queen.
Aegon had been raised in relative isolation, well-educated but with no political experience – he spent most of his life on the boat. However, much like Matthias, he will have capable administrators by his side – in particular, Randyll Tarly and Barristan Selmy are likely to join him.
During his rule, Matthias had made many enemies among high nobility, but was respected and liked by small nobility and the common folk, on whom he relied against the high nobility. While not yet fully clear, it appears that Aegon may have that tendency: his decision to appoint Duck to King’s Guard is based solely on the basis of Duck’s loyalty to him, as opposed to birth or social status. Similarly, Matthias relied heavily on Blaise Magyar, a man of humble origins but a capable soldier.
There are further similarities between Matthias and Aegon. Matthias Corvinus was a son of John Hunyadi, a commander who was well known and respected for his achievments against Ottomans. While Aegon’s purpoted father, Rhaegar Targaryen, was not that successful of a commander – indeed he made some amateurish mistakes – he is well respected in Seven Kingdoms even long after his death. Just as with Matthias, respect for the father can help build case for the son.
During rule, Matthias Corvinus relied heavily on small nobility and commonfolk to balance the influence of magnates. Almost immediately, 18-year-old king strifed with Szilagy, who soon left Budim, leaving Matthias to rule on his own. This again can be seen with Aegon, who has started acting rather independently from designs of both Varys and Jon Connington. But while Szilagy quarreled with Matthias, Connington seems quite content to allow Aegon to make his own decisions. And much like Matthias, Aegon may get distracted from real threats (Ottomans for Matthias, Others and Daenerys for Aegon) by fightings western enemies (Habsburgs / Ironborn).
Problem is that Daenerys and Aegon both have had images constructed to them, with a good layer of self-delusion on top of that. Aegon, additionally, had been protected and we are yet to see how he will handle betrayals and plots that Matthias had to handle early in his reign. Further, as much as based on real history ASoIaF is, it is still a story. Therefore, while it makes a sense that Aegon will make a good ruler in the vein of Matthias Corvinus, he is not likely to end up ruling for a long time. Best case scenario for him is a quiet retirement somewhere – much like what happened to Aegon’s likely inspiration, Perkyn Warbeck – but even that is too optimistic, as Martin’s world is much more brutal than most of real history was. More likely outcome is that Daenerys will kill Aegon, which will loose her the support of Westeros – she will become an usurper, something she always hated.