I will be able to spend 4 nights in Tuscany and 2 nights in Florence in May. I am interested in seeing Siena, San Gimignano, Chianti, Montepulciano, Montalcino and possibly Pisa. What part of Tuscany do you recommend I stay in?

dtph,

You can visit Pisa and Siena from Florence, but then that doesn't leave you much time in Florence.

Montalcino and Montepulciano are further south and there is plenty in the area to fill 4 full days. I've stayed in both Chianti and Val d'Orcia (Montepulciano, Pienza, Montalcino) and loved both.

Just remember the hilltown rule of thumb: no more than 2 per day!

Stay in or near Siena for that selection.
Look at Fattoria Tregole.

Hi dtph,
We loved our stay at Fonte Bertusi http://www.fontebertusi.it/eng/ in Pienza. They are fabulous and we felt it was a great location to visit the hilltowns. They have apts and B&B facilities. Their breakfast were the most wonderful way to start our days.

You can visit San Gim on your way from Florence to Pienza if you wish, then you can do a day trip to Siena during your stay in Tuscany. We did not enjoy Pisa, although many do. There were tons of vendors lining the walk to the Tower and just tons of tourists (ourselves included!) surrounding the tower. We were there barely one hour before leaving. We stopped en route from the CT to San Gim.

Take care,
Michele

Thanks to a maitaitom trip report we are staying at Mulino di Quercegrossa near Siena. The places you list are all reachable by day trips.
http://www.mulinodiquercegrossa.it/sito/en