I do 85% of my shopping at Longo's in Maple Leaf Square mostly out of convenience since it's close and has great hours (7 am to 11 pm. 7 days a week). The other 15% is the odd trip to Loblaws, Whole Foods, Wal-Mart, No Frills (Taal is right, the Front St store is great) and Kensington for supplements/health foods (Tutti Frutti is the best).
I've yet to go to Fiesta Farms but have heard nothing but great things. I want to start going there and utilize more farmers markets too since I'm surrounded by them during summer (CityPlace, David Pecaut, SLM)