I think Plant World is big enough to justify the "world" part of its name. It sprawls over a huge swath of land, off a rather featureless section of Eglinton near Royal York Road. They have multiple greenhouse nurseries, as well as large outdoor areas which house pretty much any domestic plant, shrub or tree that I can think of.
The prices are a little higher than the average garden centre, but they make up for it by carrying harder-to-find varieties, and they have good seasonal promotions too. It's best to have an idea what you want in advance, since it can be a bit tricky to track down an employee to help you out--especially if you come when it's busy, like on weekends or when they have sales.