Whenever I see people talk about how they want a "mean", "aggressive" dog it kind of irritates me. If you aren't first and foremost getting a dog as a companion you are doing yourself and especially the dog a big disservice. On top of that you open yourself up to a boatload of liability if the dog bites someone because they are not properly trained. This eventually winds up with certain dog breeds being banned in towns just because of the negative press that they have recieved. I actually had to switch companies for my homeowners insurance because they found out that I have a Mastiff. :barf:
Ok rant mode off.
That being said, I want a dog that alerts me when someone is at the door. If they have a big bark all the better. Knock on my door and you'll hear a 207 pound Mastiff on the other side that sounds like he wants to make you kibble. But here's the best part. If your someone I know I tell Hannibal to sit and I let you in, then the only thing you have to worry about is being kissed and slobbered on to death.
Training is the key no matter what kind of dog you want to get.