One that fits your lifestyle. For me it is boxers, but they are not for everyone. They require a lot of exercise, but are great with families. If you do a search online you can find several surveys that help you pick the right breed for your life. Also please consider a rescue. There are many pure breeds as well as mixes that need loving homes. Plus of you get an adult dog you can skip the potty train.
Mixed breeds live longer because the inbreeding of pure breeds causes many to have genetic defects sadly. Go for a smart dog in the top ten; 8 out of the ten are large dogs ( border collie, Labrador etc) the two are small breeds - papillon (little French long hair) and the poodle. I have a papillon he is ridiculously smart and relaxed and lots of fun <3
It depends on where you live. If its in the city I would recommend a small dog like a pug or mini poodle. If you live in the country or have a very large back yard I would recommend a Australian Shepard or a Great Dane. Or a Mutt :)
Pure breeds come with a lot of health problems. I always recommend to rescue a dog from a local animal shelter rather than deal with personal sellers (I bought a white lab puppy and it had parvo. It died less than a week after I got it) Shelter dogs are tested, vaccinated, spayed or neutered, and the adoption fees are far less than if you had all that done on your own. Mixed breeds are the best because they have the best of each breed. Think about your lifestyle, are you active outdoors? Have a large yard? Kids? Do research on breeds with the qualities you are looking for. I have a golden retriever and cocker spaniel mix. She has the coat of a retriever and is the size of the spaniel. She'd rather stop and smell everything in a small area rather than go on long walks. She barks like a spaniel yet has the retrievers intelligence. She is extremely loyal and amazing at following verbal commands. She's great with other dogs, cats, and kids, and really doesn't require an enormous amount of activity. Other than constantly having to take her to the groomers, I have had absolutely no trouble with her.