http://www.justdogbreeds.com/watchdog-breeds.html
A watchdog is a dog that is highly alert and will raise the alarm by barking vigorously whenever someone approaches or enters your property. Watchdogs are different from guard dogs. (The job of guard dogs is to protect you and prevent intruders coming on your property in the first place, whereas the job of watchdogs is to raise an alarm.) The purpose of a watchdog is to alert you of danger. This is a list of the best watchdogs. These are dogs that are very alert and will always bark up a storm when a stranger approaches your home or enters without you knowing. All of these breeds in this list are excellent watchdogs.
Airedale Terrier
Boston Terrier
Cairn Terrier
Chihuahua
Dachshund
German Shepherd Dog
Great Pyrenees
Miniature Poodle
Miniature Schnauzer
Rottweiler
Scottish Terrier
Shih Tzu
Silky Terrier
Smooth Fox Terrier
Standard Poodle
Toy Fox Terrier
Toy Poodle
West Highland White Terrier
Wire Fox Terrier
Yorkshire Terrier
Australian Terrier
Greater Swiss Mountain Dog
Lhasa Apso
Schipperke
Affenpinscher
Miniature American Eskimo Dog
Standard American Eskimo Dog
Toy American Eskimo Dog
American Foxhound
Canaan Dog
Chow Chow
English Springer Spaniel
Finnish Spitz
German Pinscher
Irish Water Spaniel
Jack Russell Terrier
Miniature Pinscher
Papillon
Pekingese
Petit Basset Griffon Vendeen
Samoyed
Shetland Sheepdog
Welsh Terrier