I'll have to cheat to finish this one...
Maegor I the Cruel
Aegon II
Daeron I the Young Dragon (I should have guessed)
Aerys I
Edit: Okay... after looking at the Targ family tree for fifteen minutes, I suspect you may also be looking for another king...
Aegon IV the Unworthy
It was rumored in Aegon's own time, and ever since, that Daeron II the Good was not actually his son. The rumor was that Daeron was fathered by Aegon's brother Prince Aemon the Dragonknight.
And... I also realized that my first answer of Aerys II actually goes against my own privately held theory that Aerys is actually Cersei's father... thus being the grandfather of both Joffrey and Tommen.