I once got into an argument on Wikipedia with a man from the UK who argued that Thunder Bay is in Southwestern Ontario, not Northwestern Ontario, because it is south of the half-way point between Ontario's northern and southern most points.
The east-west dividing line, Thunder Bay is the only major city west of it. 84' 41" W
The North-south dividing line. Kenora and Dryden are the only cities north of it. 49' 15" N
Almost all of Ontario's population is in the southeast quadrant.