In 3 John, who "loves to be first" and will not accept the elder's instruction?

Answer: Diotrephes

Diotrephes, who loves to be first, rejected the elder's instruction, slandered him, and put brothers out of the church.

“I have written to the church about this, but Diotrephes, who loves to be first, will not accept our instruction. So if I come, I will call attention to his malicious slander against us. And unsatisfied with that, he refuses to welcome the brothers and forbids those who want to do so, even putting them out of the church.”— 3 John 1:9-10 (BSB)