Florence may have the art, Siena has the history, and Montalcino definitely has the wine. Assisi just oozes religion. It is the home of three saints; St. Rufino, the patron saint of Assisi; St. Clare, founder of the order of "Poor Clare's;" and Italy's patron, St. Francis of Assisi. It is difficult to get excited about saints and their holy relics in this day and age. Most of the holy relics have dubious documentation, having been "discovered" hundreds of years after their previous owner ascended to sainthood from a grisly martyrdom. Saint Francis, it seems, was destined for sainthood while he was still alive and he became a saint just a few years after his death - by natural causes - though he did supposedly suffer from stigmata. His garments on display in the Basilica of St. Francis, like his mortal remains, are probably the real deal, as are those of St. Clare.
Religion aside, it is hard to top the "WOW factor" of Assisi. The former Roman city sits high on a hill and is wonderfully preserved. It is a joy to wander the streets and see the changes in architecture from Romanesque to Medieval to Renaissance.