Ah, so many choices!

Well, Shelties (obviously) because they're so perfect. They're extremely loyal and only want to please. They have such a gorgeous coat that anyone could spend all day looking at and come in a variety of colors. They're not too big but they aren't terribly small. They're extremely intelligent and will often pick up new commands in a matter of minutes. They're not a very dominant breed and generally get around really well with kids, other dogs and other animals. Kai loves kids, other dogs, cats and even let a couple rats climb on him a couple times. They're so versatile. Shelties are often seen doing flyball, agility, herding and obedience work. They're good watch dogs but horrible guard dogs. Mine would bark at an intruder, show him where all the valuables are, and then lick him to death before he left.

I also love border collies for many of the same reasons.