What's a good breed for a shop dog who's friendly with customers?

Can anyone recommend a good breed of dog (maybe medium size) to keep as a pet and shop dog. Looking for a dog that will be friendly to customers but could be a good watch dog. I'm not talking about a pet shop, but a pet to have around a shop during business hours. Anyone have good experiences with certain breeds who can be hospitable to strangers, quiet and not want to run off.