The southern side. Can you have that better? Probably. You’d need a time with no people on the streets, interesting light and good weather.
The southern side of the piazza would be fine on a sunny summer morning. That’s easy. It would also cover the western side, but I’d don’t care about it.
The northern side needs a sunny winter morning. We’d have good sidelight and certainly no tourists on the streets. The
problem would be that cities tend to have Christmas markets in such places. Just google +milano Mercatino di Natale in Duomo and see for yourself 😁
The Duomo? Pretty hopeless, I guess. In the morning it has backlight. The facade is lit from noon to sunset, and I can’t imagine the piazza being empty anytime in that interval. Probably in the evening, after a summer rain, when the sun comes out one more time. That’s what you’d have to wait for, but you can’t plan for that.
For me, visiting for one day in early September, being there in the early afternoon, there are no choices.