Chihuahua - because they are so devoted I know there are lots of dogs that are devoted, and it's very hard to choose a most favorite dog breed, but chihuahuas are my most favorite. Here's a picture of our chi Sugar.
They are so cute, can go with me anywhere (as kids we snuck our chihuahua into Gemco once and no one noticed), take up very little space, eat very little, and are incredibly devoted to their family. They would fight to the death if they thought their family was in danger.