I'm little surprised no one has mentioned a border collie yet. If you want a smart, very trainable dog with the energy to follow you all day long, a border collie is a tough breed to beat. Top choice of breeds for sports like agility because of their remarkable intelligence and incredible athleticism. Not to mention border collies tend to be snugglers, which I adore. They're also medium sized, and while I love the big dogs I have to admit a 60 pound snugglebug is easier to deal with than a 120 pound loaf. My brothers shepherd/rott can not be moved once he takes a seat, and his enthusiasm for greeting friends will knock you off your feet if you aren't seated.

I've also had German shepherds, which make for excellent family dogs due to their incredibly loyal nature and high intelligence. They're great dogs and I have known several very outdoorsy people who had them and took them everywhere (hiking, camping, you name it). Though a lot of them tend to face some pretty serious health problems either fairly early or pretty late in their life.