As a pet sitter for 10 years I have a lot of experience with many different dog breeds. The German Shepherds I have come in contact with, while they have been beautiful and I love large dogs, I am wary of them because they have been dog-aggressive.
Smaller dogs have also been dog aggressive. I don't know if this is because small dog owners just don't bother to take the time to control their dogs or what. I have owned Dobermans since '98. I have had 4. Each and every one of them has been controllable and I've had no complaints. Maybe this is because I put in the time and effort. I grew up training horses. I never stop being conscious about what is going on around me. But, I have found Dobermans easy to work with. I have found other breeds sometimes stubborn.
Rottweilers are lovable and get a bad rap like Dobermans. At one client's house, I had two Rotties in my lap at once, both purring (it sounded like they were growling). Later, I realized that perhaps _some_ Rottie_ owners who might not be experienced with their dogs were actually afraid of their Rotties who 'purred' and the owners thought they were growling. Well, that is another story. Two hundred-pound Rotties in my lap being petted were definitely 'purring.' But, I can understand if someone second-guesses their dog if they don't spend much time with him or her!
Doberman is my first choice. But, I only recommend them if a person is willing to take on the responsibility of going up to each every person that they see and saying that they understand that Dobermans have a bad reputation but please meet my dog because s/he is different (and YOUR dog HAD better be FRIENDLY!!)
Every Dobie I have had could be trusted with a toddler, so don't even take your Dobie out of your yard if s/he can't pass the Toddler Test!