GO doesn't take you to YYZ. Easiest way to get to YYZ is the airport bus and Union Station is as convenient as you can get. What you do is cross the street in front of the hotel (there's a tunnel or you can cross at street level. The YYZ airport bus leaves on the west side of the Royal York.
I'm not sure about TTC but it involves a subway ride and then a bus. There's nothing really cheap and convenient otherwise.
Airport transport info
[This message has been edited by terenz (edited 03-24-2001).]