This is old:
Toronto-Pearson (YYZ) is my favorite of all. Its observation deck is actually the top of a parking garage (although the lower floors offer better viewing, I can't stand the dirt and car exhaust), and offers 360º of viewing. Take the elevator near the entrance to Terminal 1 to the top floor, go up one more flight of stairs, out the doors, and have a blast. There are also excellent runway approaches and some nice places around the terminals where you can get very close without much hassle, even have a little picnic on the grass beyond the runway threshold of 24L!
My favorite spot: By foot, exit Air Canada's Terminal 2 and head to the right along the front (towards the International gates). Keep walking in the grass along the access road after the sidewalk ends. You'll pass around the end of the terminal and have a fantastic panorama of the back side, as well as some close up views of the end gates (including the justly famous gate 115). Continuing along the highway (crossing if necessary but, in any case, with caution), you will eventually end up at the 24L threshold, a very nice spot indeed. You'll probably find some other fanatics there. It's sort of a conspicuous spot, so you may end up speaking with the RCMP (the Mounties) or Peel airport police, but they are said to be generally pretty tolerant. I dunno, you may not actually be allowed to be there, but people always are anyway.
The sights are a little bit different up north: lots of Air Canada, Canadian, and several charter carriers for the snowbirds. There are lots of bigger jets than anywhere else in the area. 747s and A340s abound. Toronto is also one of the few airports in the world where there are Lockheed TriStars still to be seen. And Toronto has the best variety of airlines to be found nearly anywhere. Better variety is rare; Heathrow, JFK, LAX, and a few others only. There are some real rarities, even airlines that can't fly to the U.S., principally Cubana. Architecture is good, too; the new Terminal 3 reminds me of O'Hare's Terminal 1, except maybe even nicer. I just love the look of glass and steel frame construction. The attached Sheraton hotel is a great place to stay in Toronto. Get a room facing the airfield. (Undocumented tip: the hallways upstairs have windows facing the action; just try and be discreet.)
Terminal 1 is an overcrowded dump, though with Air Canada and Canadian moving their international flights in during their merger process, it will probably be getting a bit of a face-lift. Terminal 2 is about half as big as it needs to be, though immaculate. Both of these terminals are due to be replaced by a single "SuperTerminal" sometime in the next decade. This picture shows the layout of these terminals rather well.
Terminal 2, currently home to Air Canada, and, as of June 3, Canadian domestic and transborder, is rather nice inside. The Rapidair departure lounge doesn't have too many windows, though. Further down in the domestic area, there are more spotting opportunities (and some decent shops as well). I can't speak for the Transborder and International areas of the terminal, though I imagine they are decent for photography as well. However, like all Canadian airports, access to the concourse area is strictly limited to ticketed passengers. Don't expect to beg or bluff your way through. But the good news is that all the best places at Pearson are available to everyone anyway.