I vote pre-unitary because it was the period where the Italians even if divided managed to express at the best of their capacities, and despite all the difficulties they made the peninsula the most prosperous region of Europe until the period of the dynastic wars ruined all.
Aside from that, I want to break a spear in favor of the Republican period, the actual one. I know for the moment the situation in my country is grim, and there were in past dark tragedies in the recent history and other bad things, but despite all there were luminous moments as well. In 1946, despite their different ideologies, all the parties which fought the Fascism managed togheter to write a good constitution, and making of Italy a real democracy for the first time in her history, and posing the basis for the economical recover which made of the peninsula a great industrial power... this should count something in the end.