Doberman is my favorite. Incredibley smart and trainable. However, they MUST be indoor dogs, have a ton of exercise and will stick to you like velcro. A bored/cooped up Doberman is NOT a good thing.
P.S.- not to sound all preachy, but fwiw please research breeders/rescues carefully. Especially with the "protective" breeds their are a lot of bad breeders that produce dogs with weak nerves/health problems etc. A rottie/doberman/etc. with weak nerves can sometimes be a bite lawsuit waiting to happen. General rule of thumb is to find a breeder that titles their ***** in one venue or another- working, conformation (showing). There are several Doberman "greeders" that will sell you a puppy for $3000 who has championships way back in the pedigree- most puppies will have a championship here or there. But you can get a dog that's sire and dam are titled and fully health tested for less. It's like this in all breeds.
If you don't have to have a purebred, there are a ton of mixes of the breeds you mentioned in the shelters. The dog in my avatar (no longer have him, sadly) came from a shelter. He is a lab/great dane mix (we think) and he has awesome protection instincts, is great with people, not an ounce of aggression, loves kids, and is freaking smart as all get out! But, if I get another dog it will be a Doberman next time