German Shepherd. I have one and they are great. It's awesome how fast they are able to train. They are really adorable and friendly. The only thing is they need lots and lots of excersice. They are smart and are very watchful. It's a great dog.
If "big, friendly, and trainable" are the primary considerations, I'd go with a German Shepherd, Boxer, Labrador Retriever, Standard Poodle, or Golden Retriever. There are giant breeds that would also be excellent in the "friendly and trainable" aspects, but they're really big. Keep in mind that different breeds have different grooming and exercise requirements too; it's important to match the dog, whatever the breed, with your lifestyle. Also, mixed breeds are often healthier than purebreds and shelter dogs make wonderful, loving pets. Check your local shelter and/or rescue groups for a dog needing a good home.
Golden retrievers are very good family pets, as they are very friendly and fairly easy to train, Collies are supposed to be some of the smartest dog that there are, so I would highly suggest one of those if training is a main asset.