It is a personal interest, preference, or choice. It is all up to yourself to figure out and decide, taking into consideration your own interest, potential, and capability (maybe also money).
Within Ontario,U of T is generally good for graduate studies, for undergraduate studies, Queens or Western are pretty good. I heard Westearn offers schoolarship for all students, the amount depends positively on your grade.For students with good potential, Queens also offer schoolarship (and indeed, Queens is a good place to start for a future schoolarly career).
Which university is best for you really depends on what area you want to learn. Waterloo is certainly good for engineering and math/statistics, while Western/Queens are good in commerce. Overall,it is impossible to make a big mistake for going any of these universities.