How can we be sure our favorites are your favorites?
Perhaps our interests don't aline and our recommendations are just shots in the dark?
Could you give us any hints about what you like?
What are the things that most excites you about Spain?
Spain is a big country, it will also be helpful to know what part of Spain you will be visiting?
Will that be after Italy or before?
Have you looked what cities in Spain have good logistical connections with places you will visit in Italy?
How many days will you be on the ground in Spain?
From Sept, to Oct, to Nov in many regions of Spain can be vastly different weatherwise, when in Autumn will you be visiting?
You see there is so much more nuance to your seemingly simple question.
That said, I'd choose Tarragona and Cordoba, of course those are for my personal interests as I like smaller cities with specific histories and known cuisine, which may or may not be what you are looking for 😉