It is the best gelato I have ever eaten.
MIG-lio-re — three syllables, 'gl' is a palatal lateral. Stress on first syllable.
When the gelato genuinely surpasses your expectations — a sincere compliment that will delight the gelatiere.
'Il migliore... che abbia mai mangiato' uses the subjunctive ('abbia') — the Italian structure for superlative comparisons with 'mai' (ever). This is a grammatically sophisticated and emotionally powerful compliment.
Questo pistacchio è straordinario.
This pistachio is extraordinary.
Specific to the flavour — shows you can identify quality in the ingredients
Non ho mai mangiato un gelato così buono.
I have never eaten such good gelato.
Alternative phrasing — equally enthusiastic
Tornerò ogni giorno che resto in Italia.
I will come back every day I remain in Italy.
A promise of repeated custom — the gelatiere's ultimate reward
Italian artisan gelatieri are craftspeople who have often devoted their lives to perfecting their product. Many have trained at the Carpigiani Gelato University in Bologna — yes, there is an actual gelato university. Receiving a sincere compliment from a customer is the highest reward in a profession defined by quiet dedication.