Bologna, hands down.
We have been there twice, for 5 days each time, and have never run out of things to see and do.
Walking the porticos, safely away from vehicle traffic, is a pleasure. They are shady on hot days and out of the wet on rainy ones. On Saturday evening, the main street between the train station and the Piazza Maggiore closes to traffic,
https://www.bolognawelcome.com/en/blog/unesco-award-winning-porticoes
The architecture and history are fascinating. There are too many museums to count, including quirky little ones scattered around the historic center and the university area (the oldest university in Europe). The Balilica of San Petronius, the main church, has an interesting shape reflecting the rivalry between the town and the Pope.
https://www.lonelyplanet.com/italy/emilia-romagna-and-san-marino/bologna/attractions/basilica-di-san-petronio/a/poi-sig/419321/359887#
It is a huge Gothic structure (6th largest church in Europe) , still unfinished, and not particularly beautiful inside, apart from the soaring Gothic arches. But it is very interesting in that it has connections to science at a time when that was disfavored by the church. The “sundial” on the floor is a long meridian line that aided scientific discoveries and even helped prove that Galileo was right about the earth orbiting around the sun rather than vice-versa.
https://pauls-bologna.blog/2017/11/02/the-meridian-line-of-san-petronio/
On weekends and public holidays, the major north-south avenue from the train station to Piazza Maggiore in the city center closes to traffic and pedestrians spill from the slide walks under the porticoes out into the street, especially during the evening passigiata. It is like a huge block party and lots of fun (if a bit noisy).
https://commons.wikimedia.org/wiki/File:Bologna_Via_Indipendenza_2.jpg
And then of course there is the food.
For daytrips from there, Ravenna is the first recommendation. It is an easy trip by train.
https://www.ravennamosaici.it/en/
I could go on and on, but will stop here and let others state their views.
And yes, we have been to Milan a few times so I can make a valid comparison. Once to attend a ballet at La Scala. Another time to get close to our airport for an early flight. Most recently we spent 2 nights there in a very nice modern hotel close to the Duomo; climbed to the roof to enjoy the fantastic sculptures, and wandered around the piazza and the nearby Galleria. We walked to the Navigli neighborhood for dinner and took a bus back. And then we were done. No need to return, apart from passing through on our way elsewhere.
But we are always happy to return to Bologna.