BC, because they are on the coast, speak English, and have a huge amount of natural resources. They also have the best climate in Canada (not saying much).
Gonna have to go with either BC or Quebec. The Quizmaster has already given the good reasons for BC, so here's the Quebecois ones.
They speak a different language and have a more separate identity to that of the rest of Canada, so therefore the split would be easier. They contain a couple of large population centres (Montreal & Quebec City, namely) and so won't be running short on manpower. Also, they aren't landlocked, they have coast on the St Lawrence river, which is important for shipping capabilities
Ontario would be landlocked.
They speak a different language and have a more separate identity to that of the rest of Canada, so therefore the split would be easier. They contain a couple of large population centres (Montreal & Quebec City, namely) and so won't be running short on manpower. Also, they aren't landlocked, they have coast on the St Lawrence river, which is important for shipping capabilities