What type of bicycle would be the most suitable for riding in winter in the city?
I've commuted through two winters in the Pacific Northwest and I have learned how to be prepared for winter wet, snow and ice. In general, my advice:


fenders...or love your mud stripe, in fact, I extend my fenders with panels cut from milk jugs
ziplock pogies if you need them
glasses (keeps the rain out of your eyes, and car splash too)
a full sized bike with a rack really helps because you have a place to put your layers or a change of clothes
if you expect ice, which loves to live on shady, unplowed residential streets, a pair of studded tires is a wise investment (I have a pair of Schwalbe Marathon Winters)
lights: winter commuting puts me on the road before sunrise and after sunset, I need to be visible
your patch kit and pump: I have had more flats in wet weather because wet rubber cuts far more easily than dry rubber. Sharp gravel, nails, beer bottles, whatever it is, I'm more likely to get a flat in wet weather.


I have not found that tire size, or tire tread has made significant difference to me, I have learned you cannot ride on a snowy shoulder, you want to ride on the least snow or ice possible, otherwise you might just slide into the gully. Studs don't help with snow, deep tread might. For plain old wet weather, I have had no trouble with road tires, I didn't get any advantage from knobbies, in fact I replaced my MTB tires with road tires.

Good luck with your commute!