If you want a purebred dog, my very first choice would be a golden retriever. They are the sweetest things and sooo very smart. However, my favorite thing to do is go to the Humane Society and adopt a puppy. Sometimes you can find purebreds, but honestly, the mixed breeds can possess the best of both worlds. They don't have as many health problems, and, of course, they love you so much, but I truly think they know you've rescue them. They're great and I love them!!!
German shepherds. I have 2 and they get alone very well. They are always within a few meters of one another and we have nearly 3 acres! They are great dogs. But in your case it really depends on the personalities of your current dog and new dog.