the simple answer is probably the one that's nearest / easiest to get to for you. it's no good saving a few quid on your shopping if it takes you a lot more time / more petrol / bus fare to get there. also, depending on your location and available transport, consider shopping online - Tesco, Sainsbury's and Asda all offer this (others too, possibly - and it depends where you are). Yes it costs a fiver for delivery but consider the time and transport costs and you're probably not much worse off.