Ephraim is listed first because he received the blessing of the firstborn instead of Manasseh (Genesis 48:8–22). Manasseh was the older son. Normally, the firstborn son received a double portion of the inheritance (compare Deuteronomy 21:17). However, Jacob gave this special blessing to Ephraim, Joseph’s younger son.
The blessing of Moses follows the same pattern. He describes Ephraim as having myriads (literally tens of thousands) and Manasseh as having thousands. This shows greater strength and growth for Ephraim.
After Israel conquered the land and became a nation, Ephraim became the leading tribe in the north. Later, after Solomon died and the kingdom divided (1 Kings 12), the name Ephraim was often used to mean the northern kingdom of Israel.