A good watchdog can come in many different sizes and shapes. The responsibility of a watchdog is to notify its owner of a visitor or intruder, not to guard the property. Because of this, even the smallest of dogs can be a great watchdog. Think of them as alarm dogs.
A good watchdog can bring a lot of peace of mind to someone who lives alone. Plus, any of these dog breeds make a great companion for individuals or families. Alerting when someone approaches is an instinct for them, and they want to protect and notify their "family" of possible danger.
The most effective watch dog has basic obedience training. They will alert when someone comes by, but will stop barking when commanded. He should be wary of strangers, but be able to learn who is a friend. Nobody appreciates a dog that just won't shut up, or makes your guests feel uncomfortable.
So if your primary purpose of getting a dog is to serve as a watchdog, check out the breeds below to help you find the best dog for you. It's a big group, but a fun one. These guys really know how to do their thing!