That depends on the season. If it is winter, I'd suggest the bar at the top of the Manulife Centre (Bloor St. & Bay Street). If it is any other season then the Toronto Islands. The best is to take a boat tour, then you get some information to go along with the view. The CN Tower is amazing but you are so high that you are looking down on the buildings so you won't get a good appreciation of the skyline.