Any medium sized breeds, where the dog cannot physically over power you, but is still large enough that you'll feel the need to train it, that are friendly towards people and other dogs, or otherwise don't have histories or jobs associated with any type of aggression or guarding behavior. Also, highly biddable dogs are just easier to train, but overall you want a dog with medium energy, so boarder collies are out.

So, it's basically retrievers, like gun dogs, are IMO very easy. Think golden retrievers, labs, that sort of thing. Poodles are another one, these dogs are breed to be around people and listen to their owner in the presence of distractions. They are still dogs and will go off script, but they are much less likely to act out on fearfulness, the biggest cause of major issues leading to euthanasia, and generally stay close (unlike many scent or sighthounds), which makes ultimately makes them very easy to manage.

Of course, every dog is an individual, and a bigger factor than breed is environment in determining out the dog will behave as an adult. I've seen very low maintenance retired Greyhounds happy as a clam to walk 15 minutes a day, but my Greyhound puppy needs like 3 hours of attention per day or he'll go nuts and eat my shoes if I sleep in for an hour. All breeds are essentially like this, though breed can make a difference it shouldn't be seen as the final and deciding factor.