Tim Horton's... One of the few reasons I like Canada and one of the only reasons why I like the Toronto Pearson Airport. Standard selection in donuts (which is the reasons why I always come). I recently tried one of their paninis, and I must say that I am a fan!
Service is really hit or miss depending on what time of day you go. I've had a flight through here early in the morning, and the service and the slowness was almost unbearable (I just really really wanted my doughnut). On the other hand, I went here yesterday at around 8 PM and it was quick and the people were friendly.
Realistically, I will always hit up this Tim Horton's (in T1 F gates) whenever I'm flying through Toronto.