It all boils down to the overall subjective experience on the day as well as personal preferences. Personally, I've been to Italy a zillion times and I consider it my second home. I've visited 44/51 sites and I hope to be able to visit the remaining few soon. I like the ranking idea too although I'll "vote" in random batches of top 10, 20, 30, etc. I didn't visit the WHS in Campania, Piedmont vineyards and Barumini.
Top 10: Rome, Venice, Florence, Ravenna, Eolie, Verona, Siena, Palermo, Mantua, Cinque Terre
11-20: Aquileia, Agrigento, Alberobello, Pisa, Last Supper, Val d'Orcia, Assisi, Modena, Tarquinia, Ferrara
21-30: Val di Noto, Villa Adriana, Villa del Casale, Valcamonica, Pienza, Royal Savoy Palaces, Sacri Monti, San Gimignano, Urbino, Castel del Monte
31-40: Syracuse, Matera, Dolomites, Longobards, Palladian Villas, Medici Villas, Genoa, Pile Dwellings, Botanical Garden Padua, Villa d'Este
41-44: Rhaetian Railways, Crespi d'Adda, Mount Etna, Monte San Giorgio