Neapolitan. It's not really mutually intelligible with northern Italian.

You have to remember that Italy has only been a unified nation state since the 19th century. Before that, the northern principalities were alternately sovereign, or ruled by The Holy Roman Empire or by France. Central Italy was ruled directly by the Papacy, and the south was the Kingdom of Naples, alternately sovereign or run by the Spanish or the French. The peninsula hadn't been unified since the fall of Rome, and 1500 years of divergent histories produced divergent languages.