Well, here's a list of people or historical names from Randor/Adam/He-man's family tree/ancestry!
He-Ro - He came first. He was there before Grayskull stood, so he's definitely older than King Grayskull!
King Grayskull - I'm guessing he's next since Grayskull was built by him. Unless He-Ro built it and Grayskull became king afterwards.
King Tamusk - wizard king from episode The Time Wheel.
King Miro - Randor's father and former king of Eternia.
Prince Adam/He-man - our hero!
Anyone can add or something in case I forgot anybody. I know there's a queen on Randor's side that once ruled Eternia too, but she's mentioned in the UK mags only.
Also, for some reason, I think King Tamask might have come after He-Ro but just before King Grayskull. Not sure, but it feels like. Maybe I'm wrong.