Montalcino is easiest by car, especially if you want to visit wineries, Sant'Antimo, or nearby Val d'Orcia towns. The roads from Siena, Pienza, and San Quirico d'Orcia are slow in the best possible way, with frequent reasons to stop.
Without a car, use buses from Siena or nearby towns, but expect limited schedules. The nearest useful rail connections are in the wider Siena-Grosseto corridor rather than in Montalcino itself.