mutt's at the pound tend to be great healthy dogs (more genetic diversity)... but you never know what you are going to get. Based on your requirements a border collie or Australian shepherd sounds like what you want, but I will reiterate what others have said. They are high energy, high intelligence dogs. They like to work and they like order. They can get a little cooky in a chaotic environment.

Lab's and retrievers tend to have the most affable personalities which make them so popular.

Though it certainly isn't a medium sized dog, something you may want to consider is a Rhodesian Ridgeback. I recently had the pleasure of dog-sitting one. Beautiful dogs, but they are big. At least the one I dealt with had a personalty much like lab, loved to run long distances, and had very short fur. If I was going to buy a particular breed, it would probably be a ridgeback.