I would say that the two most fashionable Toronto neighborhoods are Yorkville and the West Queen West area which Vogue magazine listed as the second coolest neighborhood in the world. Yorkville is more posh while West Queen West is more hipster. I prefer Yorkville but both areas have their perks. Yorkville offers a good mix of high end and low(er) end stores as well as great restaurants.
Here is a list of places to shop and eat while in Yorkville