Tuscany Hill Towns, where to do base

Base yourself along the freeway (RACCORDO FIRENZE-SIENA), between San Donato in Poggio and Siena. That's central to everything and the freeway will give you fast access to all towns in no time.
Poggibonsi is probably too big and industrialized to be considered romantic, but Colle Val D'Elsa, San Donato in Poggio, Tavarnelle and Barberino Val D'Elsa are all nice. Monteriggioni is a pearl, but very small. Siena is big and nice, but the historical center is closed to cars, therefore you'll encounter traffic issues with a car.
If you want to stay closer to Florence, look into Impruneta or also San Casciano.
See what you can find in those areas in terms of accommodations. I don't know if you prefer a hotel/B&B type in town, in the country, or even an agriturismo (farmhouse). Do some research in the areas I told you and you'll have a nice base.
For the towns to visit, the choice is limitless in Tuscany.
www.borghiditoscana.net/eng/
If you base yourself along the area I told you, you are no more than 2 hours from every place in Tuscany, but most will be within 20 to 50 minute drive.

You might look at our trip report. We stayed in a castle about 30 minutes (not miles) outside of Florence. We thought it was very romantic although it was not a town.

https://community.ricksteves.com/travel-forum/trip-reports/nine-days-in-tuscany-what-a-deal

Let's assume that any town in Tuscany that is romantic and has a choice of dining options will also have tourists. A little more of one and less of the other is probably the trade-off. Farm-stays/agritourismos may include both a small breakfast and a nice dinner in the package price. It can also work to make lunch your main meal of the day, at each new town you visit, if your homebase has fewer options.

Thank you all!! I will have already visited Rome, Florence, Venice and Verona, thus why I want a change of pace! Not sure wich "must see" towns are (there are so many). That is why looking for a home base, romantic and easy to accses to other (Siena, Volterra, Cortona, Chianti or other wine regions, Pieza..etc) 4 days! Thanks again!

We were in pretty much the same situation in that we visited the major cities and wanted to visit the Hill Towns. We are there now and finishing up that segment of our trip. First thing we did was get RS book Hill Towns Of Central Italy. Besides Tuscan Towns we also wanted to visit Assisi. We than mapped out which towns we wanted to see. Our list included Assisi, Montepulciano, Sinea, Pienza, Montalcino and San Gimignano. Wished we could see more but just here for four days. We like apartments our hotels and found a great place in Sarteano. Don't regret it one bit. We spent full day in Assisi loved it and visited Pienza on way back and had dinner there. Siena took us the whole day because we did all of the sites at the Duomo. There are four different areas to see and loved them all. On the way back we stopped at Montalcino and walked around and bought some Brunello wine. Montepulciano very nice too. San Gemingnano today on our way up north. We keep a very busy schedule which is usually too much for most but we enjoy it. Good thing you have car you need it. Only other thing, make sure you have GPS. GPS is my best friend here! Bring yours from home so you don't have to relearn it in the rental. I use my Garmin with my Europe chip and love it.

Great advise all!!! Bought RS Tuscany and for the moment I have booked a hotel / spa in San Quirico D' Orcio, near Pienza and Montelpuciano. Going to San Gemignano and Volterra on my drive from Verona. Then I will have 4 days to see Cortona, Siena, Ovieto, Montepulciano, and maybe cruch a Winery Castle in Chianti..etc... What do you all think?

OF COURSE!!! I am from Argentina...it is in my blood!!! Still...every Tip is always welcomed!!!

We also stayed near Pienza and thought it was a great location. You mention wine tasting in your comment in chianti, I'd also recommend looking up tasting in Montalcino, as the Rossos and Brunellos are amazing. We toured three places (appt needed) and it was just us and the guide -- so personal! We really liked Montepulciano, Cortona, San Gimignano -- but really thought Civita di Bagnoreggio was pretty interesting as its been untouched by time(for the most part) and I believe it's Rick Steves' favorite hill town. You can only access by a pedestrian bridge. You can probably also hit some on your way in/out of Tuscany and use the full days to get further out.