We have 2 Boxers, 1 Doberman, and of course my Holly girl! My hubby wanted a dobie not only because he had one several years ago, and wanted to own another, but because of they being a guard dog. Our boxers will bark and act very mean but basically they will lick you to death if you come in the house. Training has been good with Bella (Doberman) so far. But anyways, here is a link, I am sure among many that you could find about guard dogs. This is the top 10 Best guard dogs, Dobermans were #2 (hubby saw and was very proud of that fact, a guy thing i reckon) so...here it is.
Top 10 Best Guard Dog Breeds