Dags. Do you like Dags?
Try
http://www.leerburg.com for protection dog forums. There is also
www.molosserdogs.com for breed specific information and forums.
First question: Are you a "dog person"?
Second question: What breeds of dog have you had?
Third question: Have you had a molossid before?
My own personal opinion is that large Molossids while quite formidable are not for the avg person or family. I speak from experience. They are my favorite type of dog and I owned a gamebred American Pit Bulldog ("Pit Bull" to most); great dog, poor guard dog, outstanding canine athlete.
Fila Brasileros have a low threat threshold from what I've seen which is a disqualifying fault for me as the breed is too formidable to be so "sharp". I don't think Presa are like that.
The issue with the Bulldogs is getting them to let go once they've grabbed something/someone in earnest. My Pit Bull required a breaking stick which would be used to pry him off of another dog that happened to get too close. He was about 73 lbs. on the chain and if he were to be matched (something I'd never consider) he would probably have gone at 65 lbs or so. I could not imagine trying to pull off a 120 lb Bulldog breed.
A "better" solution would be a well-bred working strain German Shepherd or, if you're brave, a Belgian Malinois or Dutch Shepherd. Actually, all of these personal protection breeds are potentially dangerous if left to their own devices.
I don't advocate PPDs unless the HOUSEHOLD is PPD savvy and you'd
better know dog psychology and Molossid psychology which is different in some areas (mainly by degree) again.
A good German Shepherd would be easier to train (based purely on breed characteristics, but there are many variables and exceptions - especially with high prey drive individuals)
You really don't need the high fighting drive and dominance of the working Malinois/Dutch Shepherd/German Shepherd if the dog will be a PPD. Thing is the prey drive and fight drive is greater in the Bulldog/Molossid breeds. And the Bulldog breeds are more powerful and tenacious.
Finally, a PPD breed is a firearm that can choose to squeeze his own trigger.
I'd suggest look elsewhere. Almost any 40 lb breed would attack someone who is attacking you. They are a first layer of defense and a loving companion first, obviously.
German Shepherds (make sure it has good hips) would probably make the best overall breed for most people for most circumstances. And I'd love to have a good one.
But just about any 50 lb dog breed will do if he possesses the right temperment AND proper training and socialization.
Whomever mentioned the Great Dane: Not. There are better breeds than the Bullmastiff as well. Working strain Rottweilers, Bouviers, Giant Schnauzers, Malinois and Dutch Shepherd quickly come to mind. And, of coure, working strain (primarily imports from E. Germany, W. Germany and Czech Republic) German Shepherds. But these aren't needed for what I think the majority of us need. Pick any of the herding breeds (minus the collie and Border collie) and you'll probably be just fine.
However, check out
www.boardogs.com for some superb breedings that give credibility to what sight hounds and wolfhounds and boar hounds were ONCE like - a long time ago. If you're into working dog breeds and history be forewarned: it's addicting.