Whom did Abner make king over Israel after Saul's death?

Answer: Ish-bosheth

Abner took Saul's son Ish-bosheth to Mahanaim and made him king over Israel, while the house of Judah followed David.

“Meanwhile, Abner son of Ner, the commander of Saul’s army, took Saul’s son Ish-bosheth, moved him to Mahanaim,”— 2 Samuel 2:8 (BSB)