Tafsir of As-Saffat 37:126

[The word "Allah"] is in the accusative case (nasb) as an appositive (badal) to "the best of creators." Abu Hayyan said: It is permissible for it to be an explanatory appositive ('atf bayan) if we say that the annexation (idafa) of the superlative form is pure. More than one of the seven [canonical reciters] recited it in the nominative case (raf'), on the basis that the Majestic Name is a subject (mubtada') and "your Lord" is its predicate (khabar), or that it is the predicate of an omitted subject, and "your Lord" is an explanatory appositive or an appositive to it. It has been narrated from Hamzah that if he connected [the recitation], he would put it in the accusative, and if he paused, he would put it in the nominative.

The explicit mention of His Lordship over their first forefathers is to emphasize the denial of their abandonment of Him, the Exalted, and to signify the falsehood of the beliefs held by their forefathers as well.