According to the ESV, NIV, NASB95, NASB and others, it was Elhanan.
| 21:19 | And there was again a battle in Gob with the Philistines, where Elhanan the son of Jaareoregim, a Bethlehemite, slew the brother of Goliath the Gittite, the staff of whose spear was like a weaver’s beam. |
ESV & others:
| 21:19 | And there was again war with the Philistines at Gob, and v Elhanan the son of Jaare-oregim, the Bethlehemite, struck down Goliath the Gittite, the shaft of whose spear was like a weaver’s beam. |
Even a mainstream site thinks this is a problem.
