Schwartz's is a classic.
Get #4 (the sandwich), maybe a poutine to share, a pickle for an extra kick, and a can of pop for a perfect meal.
Given the popularity of this place, lines can get very long. Best to go off hours to minimize the waits. They also have a to-go counter to the side of the main restaurant which sells the same sandwiches as well as vacuum sealed meat if you're looking to bring some back home.