There's no good advice for this one. Pearson airport is already closer to Chicago than downtown Toronto.... so if your flight is departing westbound (as many of them do), then you don't get a view of downtown from either side.

If the flight departs eastbound, the plane usually turns left (i.e. towards the north) so there's not much view there either. Best bet is to be on the right side.

But sometimes, eastbound-departing flights will do a 180 degree turn to the right, in which case the best place to be is on the left side of the plane.

If you ever get the opportunity to fly in or out of the downtown Toronto airport, definitely check it out. The views of the skyline are incredible.