I've been asked this question a few times in the past. The answer is difficult, because the best corridor will depend on the vision--referring to my opening comments--what the long-term vision is for the overall system, based on whether or not you want to service the maximum number of centres or you want to carry people from point A to point B in as fast as possible a timeframe. What you will do in between those points or at the end points will change your decision greatly.
For example, York region is a fine idea, but I'm sure Mayor Miller would have a different idea.