To find the best time in year to visit these cities is pretty hard because they are all beautiful all year long. 
If I had to decide I would go to Stockholm during the winter. I like to see the nordic cities with snow and the cold sea.
In Milan I was last december and I have to say that was a good decision because the wether was not too warm or too cold. It had the perfect temprature to visit this city. I can imagine it´s the same with Turin.
Krakow and Brussels would I visit in summer if I was you. During the winter they seemed very grey to me.
As I live in Vienna I can tell you the most about this city: The best time to visit is late spring/summer or the time before christmas when ther are many "Christkindlmärkte" in the town. There are also some good arguments for a visit in autum like fresh vine ("Heuriger") but I would prefer summer or the time before christmas.
If I had to decide I would go to Stockholm during the winter. I like to see the nordic cities with snow and the cold sea.
In Milan I was last december and I have to say that was a good decision because the wether was not too warm or too cold. It had the perfect temprature to visit this city. I can imagine it´s the same with Turin.
Krakow and Brussels would I visit in summer if I was you. During the winter they seemed very grey to me.
As I live in Vienna I can tell you the most about this city: The best time to visit is late spring/summer or the time before christmas when ther are many "Christkindlmärkte" in the town. There are also some good arguments for a visit in autum like fresh vine ("Heuriger") but I would prefer summer or the time before christmas.