Without a doubt - the Chihuahua. They are very devoted dogs to their owners and incredibly intelligent. I have a small chi and she is my assistance dog. After a stroke, I lost some of my vision and find any light incredibly painful to my eyes. I go out on a scooter because I have limited mobility. Zoe (my chi) and I go out regularly because I receive no services to help me. Obviously, with my dark, wraparound sunglasses and driving a scooter makes me look like a victim. Zoe has realized that I have difficulty seeing and has become my eyes.
Several times she has seen questionable characters approaching that I am unable to see. She barks an alert to let me know I'm missing something and then begins to bark and defend the area around the scooter.
When it became clear to one person in particular that Zoe was ready to fight and I could see them now, they left in a hurry. I feel much safer with my six pound protector with me.