And among human beings, none shall be more loved by Me than him, nor shall there be another more dear to Me than him on earth.

Priyakṛttamaḥ — one who does what is most beloved (priya). One who teaches the BG with devotion is supremely dear.

Double negation for emphasis: na kaścit — no one more. Saṅkīrtana (propagation) of knowledge is the highest service.

Tasmāt — than him, than the devotee who teaches.