I have read several messages on Toronto Delis on this list.In my opinion, by far the best one, has not even been mentioned. It is Centre Street Deli on Centre St in Thornhill.
It is related to the great Montreal restaurant Snowdon Deli and gets all of the great montreal deli meats from them. Try their Montreal old fashioned smoked meat sandwich. It is without peer in Toronto. Their french fries, cole slaw and pickles are also the best as is their chicken matzo ball soup and chopped liver.
They also have genuine Montreal bagels.
I make it a point to have lunch there every time I'm in Toronto.