Pearson is probably best - I believe Porter Airlines no longer flies to St. John's via Billy Bishop, but I'm not entirely sure about that.
Keep in mind it's very expensive to fly to St. John's. You can luck into a seat sale but otherwise be prepared to pay more than you would to go to Europe, etc. When we get a seat sale on direct flights to Dublin or London, it can sometimes be cheaper for us to get to Toronto or Western Canada by flying to Europe first.
If not Toronto, then I imagine Montreal is the best option. There are some connections between us and other cities in Atlantic Canada that are often reasonably priced (by our standards).
Like tomorrow you could get a flight from Halifax, Nova Scotia, to St. John's for $413. It's a 1.5-hour non-stop flight.