It has been a fantastic evening. Thank you for everything!
'Fantastica' = fan-TAS-ti-ca, stress on second syllable. 'Grazie' = GRA-zie.
Use this when leaving a party to give the host a warm, comprehensive thank-you. 'Grazie di tutto' (thank you for everything) acknowledges both the material effort (food, space) and the social investment. It is more complete than a simple 'grazie'.
'È stata' is the passato prossimo of 'essere' — 'it has been'. Note that for describing how an event went, you use 'essere' as the auxiliary: 'la serata è stata bella' (the evening was beautiful). 'Di tutto' (for everything) creates a comprehensive expression of gratitude.
Ci rivediamo presto, promesso!
We will see each other again soon, I promise!
Combining farewell with a commitment to future contact
Sei un/una ospite meraviglioso/a.
You are a wonderful host.
Specific compliment to the host's hospitality skills
Tornarò senz'altro la prossima volta!
I will definitely come back next time!
Expressing desire to repeat the experience
The Italian farewell at a party ('il commiato') is rarely brief. Hosts often resist guests leaving, offering 'ancora un goccio' (one more drop of wine) or 'almeno un caffè' (at least a coffee). This ritual resistance to ending the evening is a form of Italian hospitality — the message being that your company is so valued that the host wants to extend it as long as possible.