GSD is the abbreviation for "German Shepherd Dog"

They're very smart and loyal, and can be quite intimidating.

I help at the local shelter and a lot of the dogs there are of the "dangerous" type.

The only one that ever scared me was a large German Shepherd...

The pitbulls are actually very friendly, but I do think there is something in their DNA that makes them unpredictable or prone to "snapping". Pitbull isn't really even a breed, it's a blanket term for dogs that look a certain way, usually staffordshire terriers.

There's lots of options, I'd be looking at: German Shepherds (or the similar Belgian Shepherd which is more atheletic), Rotweilers, a huge Great Dane is always intimidating, Cane Corso/Presario Corso (these dogs are what I've always pictured when I imagine a demon dark guarding the gates of hell), and Dobermans. It's mostly the intimidating look, as all dogs can be protective. The strongest dog I've dealt with was just a black labrador mix. Thing was average size but it would literally pull and drag you on the leash!

The other thing to consider is if your area has BSL (Breed Specific Legislation) where there is more liability, insurance required, muzzles in public, etc. Around here the list includes "pitbulls", cane corso, staffordshire terriers, American bulldogs (but not English bulldogs).

Surprisingly the list DOES NOT include the rotweiler, doberman, or German shepherd.