There is no bad time to come to Rome, but I think that autumn is my favorite season. The stifling heat of summer is gone and the crowds dim a bit. You may have more rain to deal with, but there are also amazing sunsets and golden displays as the leaves begin to change. 

In late autumn, the starlings arrive and dance across the skies at dusk in mesmerizing murmurations. The days are short (and the sun begins to set before 5 pm), but the nights are not yet too cold.