The most aggressive by nature that I've seen are rott/pit mix and lab/pit mix. Years ago we used to keep and even raised a couple litters of pure bread colby/jeep Staffordshire terriers. Fantastic dogs, they were also raised responsibly and well socialized. Have a Presa now, and the only time you have to worry about her comin unglued is if you're messing around on the property/theft, or you show aggression to a family member. Very good with commands and will stand down if I command her too, she'll even catch/no kill and wait for me to call her off. Amazing working dogs if you use some sense and work with them from the time they are weaned. They can also be a nightmare in the wrong hands. Certain things you never do/ allow them to do, they are dominant by nature, and if you give them room to feel they have established dominance of the house, you have just invited a world of trouble in. Not at all for the inexperienced, or thug types. I would never allow someone to adopt a dog of that caliber without some knowledge of their intent, or experience with molossers/catch dogs.

I have been gashed by an aussie though. Slipped up behind me, never barked, and BAM!