Visenya Blackfyre.

Eldest child of King Maekar. Inherited at age 29 after 4 disastrous kings (her grandfather Aemon assassinated by Faith of the Seven zealot brother-in-law after converting to R'hllor, her father Maekar being severely burnt and crippled by the aztec invasion, nearly assassinated by the same man who killed his father and then becoming an insane tyrant in his quest to get revenge, her brother Viserys inheriting at age 12 and dying of sickness at age 17, and her younger brother Maelor being smothered at age 10 barely a year into his reigh) already widowed twice and dealing with the huge fallout.

Ended up being the greatest ruler since Daemon I, ushered in a golden age and set the stage for her son Aerion to begin a conquest of Essos. I had always expected she'd be heir (Maekar was a gay cripple. You have no idea how hard it was getting other babies out of him, let alone two) but it went off way better than I thought.