I've lived in Toronto, Saskatoon, Montreal, and Thunder Bay (lol).
Montreal is number 1 overall, I think. Has more if its own identity than the others. Reasonable costs of living, lots to do, beautiful city.
Saskatoon is really nice if you're cool with the small city life (250k people). Very green city, lots of space, short commutes. Very friendly people. Cons are winters are pretty brutal and the economy seems to be in a downturn now (I lived there 2009 - 2014 when things were booming)
Toronto is great, but it definitely feels less personable than the above two. Living costs are outrageous too. That said when it comes to food, events, and overall things to do, it's on top. Extremely ethnically diverse too, which is great. I've never seen a city this diverse.
Thunder Bay is...well the scenery and wilderness surrounding the city is beautiful - all of Northern Ontario is. In terms of the city itself...If you weren't born there or something I'd never suggest living there though. Lots of social and economic issues.