I hope you enjoy your trip to Toronto!
lately there are not as many good spots to spot from like there were in the past.
Terminal 1 top of old parking garage is good..(is it still in use??)
uses Terminal 3 so not sure how good it will be.
I am not aware of any spot where you will get a good view of T3
I used to go to the Wendys nearby Terminal 3 (right on approach path of the runway) at around 3-4pm and see all the heavies come in. In the summer there are some picnic tables out there..to have lunch and watch..sadly the large boeing parking lot cannot be used anymore...