I think several dogs can be considered good guard dogs but the most important aspect of a guard dog is the dogs training, you could have the best guard dog breed in the world and he will be useless if he isn't trained. Likewise a well trained "normal" dog might be a better guard dog than lets say a Bullmastiff, which is ranked number one according to a website I found.

On that same website the German Shepherd is ranked number 7, for me a bit of a disappointment. I think German Shepherds are very effective guard dogs and besides that they can still be great companions and very playful animals. The page rightly states that the dog is very intelligent (apparently the third most intelligent in the world) and this also reflects its use as a police dog in many places in the world. The fact that these dogs are smart also makes them easier and more effective to train, adding an extra advantage.

In my experience the German Shepherd is also very loyal and that is definitely a crucial characteristic for a guard dog. Although the German Shepherd does not look very intimidating it can become very aggressive and protective, which is desirable to fend off wrong-doers.

i would have to say the Lapaha Blue-Blood Bulldog... http://dawgforums.com/forums/t/4150.aspx

Rottweilers are good guard dogs because of their alertness and wait-and-see attitude. They do not usually bark unnecessarily until and unless they see any harm is about to come upon their owners or the territory that they are guarding. They are quick learners but nevertheless, need firm handling and discipline due to their strong-willed nature.

I drive for FedEx and see dogs daily. German Shepherds seem to make good ones even though one bit a hole in my leg. Boxers alert quickly when I pull into a driveway but they are twitchy and may just eat you insead of a bad guy.