Toronto and Montreal are both hubs for international students as they provide a great city atmosphere as well as a great reputation as far as academics go.
Toronto is a powerhouse for universities and colleges alike. There are many prestigious schools very close in proximity. Surprisingly, they each hold a niche in different academia that makes them all very appealing in their own ways.
Montreal is most commonly known for McGill University. Montreal is an interesting city because you get a taste of both English-speaking Canada as well as French-speaking Canada. Montreal is a very hip town and there are always many things to do and get involved in.
I would suggest looking outside of these cities centres as well though, you’d be surprised by the many schools that hold just as much prestige. Ontario has many universities close together, but don’t be shy to look into schools along the west and east coasts as well. They have a lot more scenery and just as much value to lend to your educational growth as the cities would!