Hands down, SR222 was the best road I´ve ever ridden. Beautiful curves through wine and olive country. Plenty of other riders, but it doesn´t feel crowded. Most of the way is sleepy countryside, but as you get into and beyond Impruneta, it gets busier as you get back towards the Florence traffic. Highly recommended.
I used this route on the way from Greece to the UK. It is a great road and would recommend it if you are in the area. I would also suggest carring on down from Sienna to Viterbo on the S2. The road is excellent.
I have only used it once, in the trip above, but it stands out as the best road of that trip for me.
A jaunt through the Chianti foothills of Tuscany. Views of ancient Medeival villages and vineyards dot the countryside. Surprisingly good tarmac with plenty of curves. Take any of the side roads and they're bound to be just as enjoyable.