Thank you for everything, it's been wonderful!
'Bellissimo' — bel-LIS-si-mo. Four syllables, stress on the second. Absolute superlative of 'bello' — expressing maximum beauty/wonderfulness.
Use when leaving after being hosted or after a particularly enjoyable experience — dinner, party, visit, trip. It's a gracious, appreciative goodbye that acknowledges the host's effort.
'Grazie per tutto' (thank you for everything) is a generous comprehensive thanks. 'È stato bellissimo' (it has been most wonderful) uses the superlative of 'bello' — 'bellissimo' (very beautiful/wonderful). This combination closes a visit warmly and generously.
Siete stati fantastici!
You've been fantastic!
Plural — thanks a whole family or group of hosts. Very warm endorsement.
Non dimenticherò questa serata.
I won't forget this evening.
Memorable farewell — commits the event to lasting memory
Avete fatto di tutto per farmi sentire a casa.
You've done everything to make me feel at home.
'Sentirsi a casa' (to feel at home) — the highest compliment to Italian hospitality
Italian hosts deflect thanks with 'il piacere è tutto nostro' (the pleasure is all ours) or 'figurati' — Italian hospitality culture requires the host to frame the guest's presence as a gift, not a burden. Accepting thanks directly without deflecting would seem arrogant.