Final answer:
The teres minor is the most superior muscle on the axillary border of the scapula, situated above the teres major and distinct from the serratus anterior, which is on the anterior thorax.
Step-by-step explanation:
The muscle that is most superior on the axillary border of the scapula is the teres minor. This fact can be confirmed by understanding the anatomical arrangement of the shoulder muscles. The teres major is inferior to the teres minor and extends the arm, helping in adduction and medial rotation. Meanwhile, the teres minor is the longer muscle, located above teres major and it facilitates lateral rotation and extension of the arm. Contrarily, the serratus anterior is not positioned on the axillary border of the scapula but rather is found on the anterior side of the thorax, and thus is not a contender for being the most superior muscle on the axillary border of the scapula.