In Jotham's parable, which plant finally agreed to reign over the trees?

Answer: The thornbush

The olive, fig, and vine all refused; only the thornbush (KJV: bramble) accepted. Jotham told it to warn Shechem about Abimelech.

“Finally all the trees said to the thornbush, ‘Come and reign over us.’”— Judges 9:14 (BSB)