I highly recommend a Sheltie. They are gentle dogs that take their guard dog roles very seriously. Ours watches all the doors, wakes for any unusual sounds, and includes anyone in the house as part of her little herd. If I walk outside or appear outside any gate, Precious is up and growling a warning. (Her method is to bark and growl first, then see if the intruder is friendly.) Strange men elicit loud continuous barking. She even flips open the shower curtain to check if anyone is behind it (we leave it closed most of the time). She is our little cop.
Once we had two electricians in the house. After they were gone Precious walked through her territory with her lip curled and a gutteral growl coming out. My wife always feel safe when she is alone with the dog. If Precious is quiet, all is well.
We have a Shelty, and she is a great watch dog. I concur. In fact, I give you a Shelty Bump!! Ruf..