Here are my top five:
Venice, by a long shot. No other city in the world compares to it.
Maremma in Tuscany (shown in the inset), especially the upper Maremma south of Livorno, including the towns of Bolgheri and Castagneto.
The Euganean Hills near Padua, virtually unknown to Americans, distinct from any other hills, but reminiscent of Tuscany.
Lake Garda, which I prefer to the other Northern Italian lakes. Also not well known to Americans.
Alto Adige or Suedtirol, the bilingual area in North-Eastern Italy. A year-round destination, in my opinion preferable to Switzerland and Austria, and cheaper. More on these later.