Whose seed did God promise would become the greatest nation on earth?

The seed of the Jared and his brother

The Lord did hear the brother of Jared ... and said unto him:

I bless thee and thy seed, and raise up unto me of thy seed, and of the seed of thy brother, and they who shall go with thee, a great nation. And there shall be none greater than the nation which I will raise up unto me of thy seed, upon all the face of the earth. Ether 1:40-43