In the Arthurian legend, it is almost always his illegitimate father Uther who was king before him. In most legends, Aeuralius Ambrosius was king before Uther, and before him it was Vortigern.
Most people believe it was King Uther, Arthur's father.There are many fans of these stories that study on Wikipedia pages for hours if they're really interested. And some who have creative urges...