....and his songs were a thousand and five.
1 Kings 4:32
What was his favorite? (Hint: the one he left us)
The song of songs, which is Solomon's.
Song of Solomon 1:1
Solomon had only one son that was recorded (Rehoboam). His mother was Naamah. Rehoboam became king when he was 41. Solomon reigned 40 years so he must have married Naamah young, long before his "other" wives. So she likely was the "love of his life". Quite possible the Song of Solomon was written for her.
Naamah means "pleasant one". Behold, thou art fair, my beloved, yea, pleasant:
Song of Solomon 1:16