CostCo is fantastic if you can deal with the crowds. Their meat is always top notch. So are their frozen vegetables. Dairy products are also crazy cheap. Half & Half is like a third the price of the regular supermarkets.
I usually shop at Longo's, which is a regional chain here in Toronto. Owned by this Italian family. They're a little more expensive but the service is superior and I find they have better quality produce.
Used to shop at the A&P (now Metro) but they moved to the other side of town. Found they went downhill hard after that. Really low quality meats and vegetables, which is inexcusable for such a big chain.
Loblaws is the big hypermarket here. They're trying to take on Walmart. My biggest gripe with them is their stores are way too big now. I like to just get in and out. Their service also stinks. They don't offer plastic bags or boxes. If you forget your canvas bags, you're screwed.
Sobey's, the other big chain, has decent stuff but they're really expensive compared to the other grocers.
Walmart of course is Walmart. It's the same wherever you go. Their grocery section is okay but I wouldn't do a full shop there. Their canned goods are decent though and usually pretty cheap.