The best option is to park at the Viscount parking garage or the cheaper open air parking lot beside it. You jump on the monorail to Terminal 1 & 3 from there. There are cheaper options but they are way off airport.
For $90/month, Skypark, is a good choice. Drive to the site, which is close to the airport, fill in the paperwork including your flight details and a shuttle will take you to the airport. On your return, call them and a shuttle appears soon, and back at the office, your car is ready, motor running and cabin is warm. Maybe not as convenient as Viscount, but much better than ParknFly. SkyPark.ca
Another shout out for Sky Park. Excellent service. Never a wait for a shuttle. I would advise booking Valet parking. In the past it has been the same price as their other lot but you get priority pick up. For our winter trips we use Aeroport Taxi and Limo. Another excellent service. We live in Burlington. I don't think they go further west.