I mostly do my shopping in Bónus - a chain of supermarkets you'll find all over Iceland. You can be reasonably sure that there you'll get the best prices over all. You can study the locations of their shops if you like, especially if you are driving or travelling around Iceland.
Opening hours in Bónus are mostly as follows:
Monday - Thuesday: 12 - 18:30
Friday: 10 - 19:30
Saturday: 10 - 18
Sunday: 12 - 18
