Jehovah's Witnesses believe that Jesus was the Archangel Michael before the incarnation so the answer would be that it would have been Michael (pre-incarnate Jesus).
"Likewise, the Bible indicates that Michael is another name for Jesus Christ, before and after his life on earth." JW.org
"The foremost angel, both in power and authority, is the archangel, Jesus Christ, also called Michael. (1 Thessalonians 4:16; Jude 9) " The Watchtower, November 1, 1995).
Also, according to JW's, the only time recorded in the Hebrew Scriptures that Michael appeared was in Joshua 5:13-15
"before the siege of Jericho, it was doubtless Michael, the “prince of the army of Jehovah,” who appeared to Joshua to reassure him of divine backing" JW.org
Outside of this they would consider the other appearances to have been an 'angelophany' since Jehovah is spirit and not man, dwells in unapproachable light, and whom no one can see or has seen (1 Timothy 6:16). If it were possible for Jehovah to become human that would mean that the theology and divinity of Jesus was indeed possible.