Rule reminder
Italian uses several fixed emphatic exclamations: 'Figurati!' (Imagine! / Of course! / No way! — context-dependent), 'Macché!' (No way! / Absolutely not!), 'Mica male!' (Not bad at all!), 'Ma dai!' (Come on! / No kidding!), 'Mamma mia!' (general exclamation of surprise/stress), 'Già!' (Exactly! / Right!), 'Meno male!' (Thank goodness!), 'Figurarsi!' (formal variant of Figurati!). These are frozen pragmatic markers that carry strong emphatic force in conversation.
A colleague finishes a project ahead of schedule and under budget. You respond enthusiastically. Which is most natural?