Which Town in Tuscany?

I've seen similar questions about this in the past, and there's always a variety of opinions.

I stayed in Siena and used it as a base for exploring the hill towns. This was great for me. Siena is wonderful in the evening. But I'm sure many of the hill towns are also lovely in the evenings when the crowds have dissipated. But Siena is a bigger town with a lot to see; most of the smaller towns, such as those on your list, can be experienced in a few hours each.

Parking in Siena can be a challenge, though, so that's a consideration as well.

I don't think you can do this wrong. Ask yourself how you feel about pulling up stakes and relocating versus staying in the same place every night. Whatever decision you make, you'll surely enjoy all the places you visit.

Siena is a great base and u can travel by bus easily to San Gimi, Montepulciano , Pienza. In fact it is much easier by public bus. Very clean with gd schedules. If u are coming in from Florence again the bus is the best option. Pick up your car when you are ready to leave Siena if u decide it could be a gd base.
Avis has an agency there outside of walls. There are many gd reasons to base in Siena. Sights, views, walled, great food, cathedral, walk everywhere with delight. A great stay.
A car due to parking can really be a nuisance imo. I have rented a car for umbria and for villages edgeing on tuscany. Many like to stay in an agriturismo when in tuscany there u would have ample parking , poss a pool and poss a host that will wine and dine u , if that is your preference there are many to choose from.

We stayed at Hotel Minerva in Siena, just outside the wall with onsite garage parking. some rooms have views of Siena's skyline. Ten minutes' walk to the Campo. We visited Monteriggione, Volterra, San Gimignano, Montepulciano, Montalcino, Arezzo and Sansepolcro (we were there for six nights).

How long will you be there? Do you want to pack up and change hotels often? Personally I like spending a couple of nights in each town, but it's not always practical.

IF you choose Siena, look at Frances Lodge - it is a b&b right above Siena so you can walk into the town at night ! Lovely !

We love Cretaiole and Borgo Argenina - eating breakfast overlooking the vineyards and driving ALL over the area is just heavenly !

The towns you mentioned are all within striking distance. If you stay in a central location in or near Siena, you will be within 30 to 60 min drive from all. Changing hotels has the disadvantage of taking a lot of your time with the packing, unpacking, checking in and out paperwork and getting oriented at the new location. When distances are so short, it's more time efficient to use a central base and take day trips. If you want to stay in a bigger town, Siena is bigger. Otherwise you can stay in a hotel or villa in the surrounding countryside or in a smaller town, like the ones you mentioned, or Colle Val D'Elsa, or Monteriggioni, or many others.

I would suggest keep one base and do day trips. Going in and out of hotels takes time. Personally we loved san Gim. I thought it would be a good base and spend our few days visiting other towns, but we were so in love with it we (2 adults and two kids 13 &10) spend nearly the whole three days exploring. We did a drive to Volterra, but were quickly bored and return to San Gim to explore more. We stayed at hotel Bel Soggiorno, Not cheap (we had a very tight budget) but it was great. Both kids have said it was their favourite place in the 6 weeks we spent in Italy, France and England.

There are many agriturismo in the Siena area. Since they are in the countryside a car is generally needed so there is parking. Many of them will offer a home style dinner to their guests which can make for a very delicious, fun evening!

Split it up your base into 2. I loved Volterra that I would stay there, and for the other I would stay in Siena. You can visit San G on the drive from volterra to Siena.