This question has probably been asked a million times, and it seems like everyone has a different answer (which is obvious, since if everyone had the same answer, most of the grocery stores would go bankrupt!).
I buy everyday things at the Yonge/Eg Metro (formerly known as Dominion) and it's an absolutely horrible shopping experience there. Product choice is limited (I still can't find more than 1 brand of Korean noodles). Aisles are tight. And worst of all, the checkout lines are a headache. Always long. Add to that the fact that you don't have that much space to line up, and during busy hours, that turns into a nightmare.
But I go here because it's only a block away from my apartment.
Sometimes I go to the healthy butcher, but not often. The prices are rather expensive. One piece of organic chicken breast costs around $8.
Once or twice a month I make a short trip with my car to the No Frills on Avenue between Lawrence and York Mills. All the everyday and regular things (ketchup, canned food, salt, etc.) I buy from there.
I also go to Whole Foods once or twice a month, especially for meat, poultry, and my yogurt/kefir products (STONYFIELD is my new discovery. What a delicious yogurt, but the search for the best plain yogurt still continues). After the CEO of Whole Foods wrote that biased and uninformative article on public health care (as part of the U.S. health care debate), I decided to stop supporting Whole Foods.
Seafood I buy from the Yonge & Yonge Loblaws just south of York Mills, or sometimes from Metro (nothing special). I've heard great things about a seafood market called Diane's or Diana's, which is either on Lawrence or York Mills close to Warden in Scarborough, so I might make a trip there sometime this week.
As for price, the cheapest is obviously No Frills. Then Loblaws (although I'm not sure whether Yonge & Yonge is more expensive than other Loblaws or not, but I only buy fish there), followed by Sobeys, Metro, Whole Foods, and then The Healthy Butcher/Pustaeri.
I have no idea about Longo's, Galati, and Highland Farms. If anyone has any input on these stores, including a price comparison, it'll be great if you can post it here. To be honest, I'm not really happy with my grocery stores in general. Nothing awesome about Metro. No Frills has a limited range of products (PC vs No Name, although some PC products are great and irreplaceable). I hate Metro. Sobeys I prefer the most, but rarely go to any of their stores.