I search google.it for la canzone d’amore più bella (the most beautiful love song) and get 741,000 entries.
The first is a YouTube post of “Adagio,” sung by Lara Fabian. With more than 3 million viewings, it merited a listen from me. I found the song a tad overwrought—more longing than loving–but seems as if I may be in the minority.
The second link is a blog called clo&demk that couldn’t limit itself to just one melody. Its Le 200 Canzoni D’Amore Più Belle di Tutti i Tempo (The 200 Most Beautiful Love Songs of All Time) leads with “I Say a Little Prayer” by Aretha Franklin. Sorry, it’s a sweet song but not Italian.