Yam, God of the Sea of the Canaanites
Mainly because he has one of the most interesting histories behind him: In the Hebrew Bible God gets constant praise for destroying "the Sea" (Yam יָ֑ם), which led to weird translations for centuries (that still persist) until modern archaeology, especially in Ugarit, revealed that Yam is actually a deity in the Canaanite pantheon and it was supposed to mean Yahweh crushed his rival deity Yam.
In Ugarit we found the Baal-Cycle, in which the major God of the Canaanites Baal fights Yam and his army of sea-monsters.

For being a minor deity with only a small cult following him, Yam challenged both Yahweh and Baal.