Since I moved in the area last April, Longo's has been my new go to grocery store.
It is located in the basement of the Maple Leaf Square and the space is very massive for a downtown grocery.
Even if it is underground, there is sufficient lighting to make it look pleasant and not creepy. The aisles are large and the sections are well organized. There is a parking attached to it that is free if you buy over $20.
They do offer most of the national brands plus their own house brand that I find quite fantastic in terms of pricing and quality.
If you are in a hurry or just at a nearby hotel, they do offer a wide variety of freshly cooked meals and wood oven pizzas and ready to eat food.
Service is great and pleasant. You also can register to their point system and gives you 50 points for every $25 you buy.