What is the best self defense dog?

It is impossible to suggest a breed as what would be easy for one person to manage, would present a real challenge for someone else. Consider breeds typically used for what you want in a dog, & then make an informed decision based on what you learn from researching what the breeds need from a home.

A Dobermann is NOT naturally protective & will given a choice, would instinctively watch its own back NOT its owners.

Personal Protection [the job the Dobermann was created to do] are learned behaviors with the suitable dogs trained to a high standard by someone with the necessary expertise, so the dog will reliably protect & do the job it was trained to do, & the time it is needed to do it.

It does however by nature typically make a good watch dog & will bark to alert is owner when someone approaches the property, but that should not be confused with guard or protection work

If a person intends to own a male Dobermann, male/female is the best combination. The reason why being it is a dominant characteristic of the breed for male Dobermanns to be same sex aggressive.

357 mag. and sec. sys. GSD to warn first then use the other two. don't put a dog in harms way, they can be shot.

No dog is guaranteed to protect you.
If your lonely, just go to a shelter and adopt a dog you like.
A trained dog costs $1500+, a gun costs $300 give or take

1

a loaded gun

If you are being attacked by a dog, yes you can hit it all you want however, when a dog is in what they call the "Red Zone" enough to attack then they feel no pain what so ever. Chances are when it's attacking you won't be able to hit it. If a dog runs at you, do not run, this makes the dog chase even more, simply stand straight up, cross your arms and do not make eye contact. Your demeanor tells the dog you mean business it's the body language that tells the dog to back off. Another method is if you are being attached you pull yourself into a tiny ball as tight as you can and you yelp like a wounded puppy, it might take a few minutes for this to register but a dog is more likely to back off than if you are fighting it. If this dog is getting loose and is attacking people then it should be reported to animal control, no dog should be running around loose especially if it's a menace to society.

You may not need this level of protection, as it usually does not take to big of a dog to make most people think twice. But if you do, the Boz Guregh has to be at the top of the intimidating list. Larger than most Mastiff, and far more capable. The Gladiator of the dog world. Bred for thousands of years to be a Family Protection or Livestock Guardian Dog. Comes natural for them.
And few dogs could have your back this good.

german shepherd and rottweiler

Unless you can afford a private trainer who take around 10-20$ per hour for about a year dont buy a dog for self-defence a dog is not a weapon!

Buy a gun and a security system, want a companion? buy a golden retriever.

But if you are prepared to pay for the training and will be responsible buy a german shepherd as their aggression is controlable and can easily be ordered to stop, and will save you money on training as they usually learn the fastest. (after poodles but i mean aggression) You also have to teach the dog obedience with the trainer for AT LEAST 2 months before thinking of aggression or the dog will be out of control and you will get sued and the dog put down.