Although there are breeds known for their ability to be excellent guard dogs, no dog is born as an automatic guard dog. To get a true guard dog, it must be trained to do the job properly which includes the most important aspect -- listening to the direction given to them by their owners.
Generally speaking, most breeds will be protective of their territory and their (human) pack. There are many, many breeds that are good with smaller dogs and children. It is very hard to pinpoint one as the best. If you wait long enough, you will probably get a lot of people weighing in to tell you what they believe is the best dog to get. I believe this is truly a personal choice. It all really depends on what kind of personality & temperament you feel will fit in well in your home. There are a few web sites out there that will help you determine what breed will be the best fit based on your answering a number of questions. Here is one of them that should give you a good start on your search:
Animal Planet :: Guides :: Dog Breed Selector
I would suggest after taking the test and seeing what breeds are best suited for you and your family, that you do a search of those breeds of dogs that are available for adoption through
Petfinder.com: Adopt a pet and help an animal shelter rescue a puppy or kitten.. Most of those dogs have been temperament tested and if you work with one of the shelters or rescue operations that you find online, you will have a higher degree of success finding a good match for your family, rather than buying a puppy from a backyard breeder or pet store. Those options cannot guarantee temperament or freedom from medical problems later as they age. As I am sure you are aware, millions of dogs are euthanized every year in the U.S. due to lack of homes. If they don't get adopted within a certain time frame, shelters must put them down to make space for the newer additions. So, in addition to knowing up front the temperament of a dog you choose, you will also have the satisfaction of knowing that you have saved a life.