I must give props to the bus #7 Airport, both North/South! This stop is located at the City Centre Bus Terminal (Square One) and it takes approx. 40-45mins to reach Terminal 1. I *think* a ride costs about $3.25.
Being a driver, I haven't had to take the bus for decades. However, if I'm going on a trip, I find this particular route very convenient and more cost effective than Park 'n Fly (at least $10/day) OR a taxi ($50-60 for a 20min ride). Bonus points if you have someone drop you off at the terminal!
Works well if you plan your route properly, re: check the bus times, aware of how long your trip will take, and arrive with enough time to check-in at Pearson.
What is not so great is that the bus stop is outdoors and the indoor shelter is a bit far to wait inside with your luggage. Not so great if you're travelling during the colder months! Also a limited number of seats where you can sit with your luggage.