Located in the Food Court of the Bay-Adelaide Centre, this Mexican fast-food joint makes some exceptional burritos. They are well priced, fresh, and taste very authentic (I believe) - it helps that all (most) of the staff working has an amazing Latino accent.
Watch out for the line up there, which starts around noon and lasts until 1pm.
I would give them an extra star if they didn't charge you 99c for only a little bit of guacamole.