Hi there! Finally, I'm back on the forum!!! Hopefully not too late to answer to your post.
I wanted to say that since you will practically be my neighbour (as you will be staying in Casole d'Elsa), I hope you will come see me at our new restaurant in Colle Val d'Elsa named "Officina della Cucina Popolare" www.cucina-popolare.com . We're located in the old part of town and will be opening next week! (so exciting!!!)
If you are interested in a wine tour during your stay, shoot me an email at this address: email@example.com
And now to answer your question of which winery I would visit if I could only visit one... oh, how difficult it would be to choose! I would probably visit a winery at which I would be given a private tour with the owner who would generously waste his or her time with me while popping open lots of bottles of wines...