Solomon
Ecclesiastes 1:2ESV·traditional attribution

Vanity of vanities, says the Preacher, vanity of vanities! All is vanity.

Matthew Henry Presbyterian @wholebiblehenry

Here is, I. An account of the penman of this book; it was Solomon, for no other son of David was king of Jerusalem; but he conceals his name Solomon, peaceable, because by his sin he had brought trouble upon himself and his kingdom, had broken his peace with God and lost the peace of his conscience, and therefore was no more worthy of that name.

Commenting on Ecclesiastes 1:1-3

John Gill Reformed Baptist @doctorgill

Vanity of vanities, saith the preacher,.... This is the preacher's text; the theme and subject he after enlarges upon, and proves by an induction of particulars; it is the sum of the whole book; vanity of vanities, all is vanity; most extremely vain, exceedingly so, the height of vanity: this is repeated, both for the confirmation of it, men being hard of belief of it...

Jamieson-Fausset-Brown Reformed @jfbcommentary

The theme proposed of the first part of his discourse. Vanity of vanities--Hebraism for the most utter vanity. So "holy of holies" (Exo 26:33); "servant of servants" (Gen 9:25). The repetition increases the force. all--Hebrew, "the all"; all without exception, namely, earthly things.