Jesus Christ
Verily, thus saith the Lord unto you, my servant Joseph Smith, I am well pleased with your offering and acknowledgments, which you have made; for unto this end have I raised you up, that I might show forth my wisdom through the weak things of the earth. Doctrine and Covenants 124:1
The whole revelation is spoken by the Lord Jesus Christ to Joseph Smith. Every 'saith the Lord' and 'saith the Lord your God' that follows in this section (for example vv. 15, 32, 50-54, 69, 71, 101, 119, 135, 145) is His voice.
How we know: the verse itself names Him
