The Song of Songs, which is Solomon’s.
The Metzudas Dovid cites I Kings 5:12, that Shlomo composed 1,005 songs. Our verse here is meant to tell us that Shir HaShirim is the song par excellence of all those composed by Solomon. The reason for this is because of the beautiful and powerful way it depicts the strong bond of love between G-d and the Jewish people. Therefore, this is the most significant song out of all the songs written by Shlomo.