Toronto is the center of the business world in Canada, and most major companies are based there. It's a sprawling metropolis, reminds me of silicon valley: Between Hamilton and Oshawa it feels like a single continous city. Toroto has lots of ethnic flavour, and strong communities, and its share of big city problems. It's close to the US border -- crossing US borders at the airport is a breeze since all US border guards are well versed in the business travel routine. In fact, most likely most business dealings from Toronto to anywhere in US is probably better/easier than anywhere else in Canada.

Vancouver is on the west coast, and influenced by the mindset frequently found in US west-coast states and cities (Seattle and SF in particular). Its own mindset is somewhere between SF and Toronto.

Ottawa has strong software community, but it's not nearly as big as the above cities (it's located 2 hrs driving from Montreal, and 4 hrs from Toronto). It's got a heavy load of federal government agencies and money.

Montreal is more affordable than Toronto or Vancouver. In my opinion, it's one of the best places to live in Canada, though being located in a francophone province with a strong independent (some say chauvinist) streak, may add extra hassle for you as a business owner (due to legal language requirements, etc). Anglophones in Montreal have a love/hate relationship with the city/province -- all rooted in history and politics. But, guaranteed, if you embrace the experience living in this de-facto bilingual city, you won't regret living there.