what visa will i need to apply in Toronto to visit mainland China?

Smiley, I assume that you don't have the Home Return Permit, right? HK residents can't go to mainland with their passports only. They need to bring their Home Return Permits. You have two options. First one is fly back to HK and apply for the Home Return Permit. Then you enter mainland from there. In the future, you can still use your permit to travel to mainland from Canada.

Your second choice is to apply for a China Travel Document from Chinese consulate in Toronto to finish your trip first. Then you can get back to HK to apply for the Home Return Permit.