I think Italy is a great place to retire for a lot of reasons and each region has its advantages and flows. I think before making a final decision you need to consider your interests, activities and, of course, finances. M.Hunt in his reply mentioned Tuscany, Umbria and Marche, 3 great regions but there are 17 other to choose from each one of them wonderful as well.
The following suggestion are based on my experience of having lived in medium size cites of the north but have traveled throughout Italy and are more general in nature.
Regarding interests, I assume gastronomy is very much high on your list. My experiences having traveled from the toe to the to top of the boot, has been great no matter where I was. Customs varying from region to region and , at times, even withing a region dictate gastronomy. This does't mean that you will not find a good pizza in the north. It is just that I follow the golden rule of " when in Rome, do as Romans do" . Culture events will be more available in large (particularly in the North) cities.
Regarding activities, the Alps bordering the top of the boot and Apennines running down the boot offer great hiking and snow sports as well as unforgettable views . I resided in Lecco and Verona in the colder months due to my love of skiing and hiking which were easily accessible from both of those cities. If you like the sea, I like the west coast more than the east and of course the islands. Having said this I will be spending the summer in Ancona because I couldn't find an apartment on the other coast matching my needs. If you like sailing, the northern portion of Sardinia and the Aeolian Islands are a must.
Regarding transportation, a car is indispensable if you decide to reside in a small village or in the outskirts of medium size cities which are not on the main rail or bus lines. The west coast is better served by rail service than the east coast . In the interior , the north has better transportation than the rest of Italy. My decision to reside in Verona and Lecco was driven by my decision not to have a car and these cities being well connected to the rest of Italy by air , train and bus services. One note, there are also bus companies serving Italy (i.e., FlixBus) . These offer great deals connecting large, medium and small cities. So, when considering a place to reside , you will definitely to consider transportation even for your weekly or possibly daily grocery shopping.
Regarding Finances, the real estate in the northern section of Italy is more expensive than the rest of Italy due to its industrialization. Conversely , the southern section is the least expensive. Of course, real estate in areas heavy with tourists will be more expensive anywhere in Italy.
A couple of other matters that you may need to consider prior to retire full time in Italy. Moving to and residing in Italy is not a smooth process (think bureaucracies) and I heard that you must prove you are financially able . Also , taxation on income may be an issue as well as health insurance (by the way on this matter, healthcare in the north is better than the south from a personal experience).
As I mentioned, my suggestions are very general in nature and are based on my experience, Others may have different views based on their experience. But I think everyone will agree that Italy is one of the best country to retire to. Hopefully, the above helps.
Buona fortuna