What is the best Pet Dog?  

Honestly, I've had experience with both Goldren Retrievers and Yellow Labs and both are extremely good dogs. They're generally easy to train, very friendly, but will definitely serve as protectors to their owners should the situation arise. Both breeds would be personal top choices.

A German Shepherd would be a good choice, they excel at both guard duties and as an affectionate companion

all the best urban

Well there isn't just one breed that tops them all--it all depends on you and your wife. What exactly are you wanting from this dog...you mentioned protection, that's a start--but do you want your wife to have a jogging partner, do you want a couch potato? Do you want a dog that will cuddle or a dog that will be on guard all the time??

TO go off what we have so far, some good guard dogs are German Shepherds, Rottweilers, Saint Bernards, Doberman Pinchers....Great Pyrenees (beautiful dogs!)

I noticed you said you are planning on buying dogs...I don't know if you just meant 'getting dogs' by that or if you are actually planning on purchasing from a breeder...but I always have to say this to people who are planning on getting a new pet(s)...please ADOPT, don't buy!! You can find ANY kind of dog you can imagine (purebreds and mixes) from shelters and breed specific rescues.

I hope I've helped!!!