Get a Shepard -get the female runt of the litter. They're usually very nice natured, very affectionate. When she misbehaves I need only say her name and she knows she's done wrong. But she will protect you- 80+lbs of canine isn't something people ignore.
I use to work nights and I'd come home park in the street, my wife would be outside reading on the porch with the dog. As soon as I'd pull up the dog would get up and stand between me and my wife- haunches up till she recognized me.
First time I heard her bark was just after I threatened to sick (perfectly on cue) her on people that wouldn't leave my doorstep trying to sell me stuff (might have even been Schwans, I don't remember). She was almost a year old.
Never put much into training her either she knows our yard and will even herd the kids if given the chance. Heck, she's smarter than half the people I know (not sure if I'm insulting her or the people....).
<edit> might I add that at night she sleeps in three places. Right in front of the front door (or on the couch next to it---bad dog), in the hall between the kids rooms, or right in front our bedroom door. Without training she's set up her own sentry points.
I am afraid this may be the best idea yet!!!!! 
