Which breed would be the better guard dog?

There have been studies shown that any dog can deter people, though obviously "scary" breeds like Rottweilers, Dobermans, German Shepherds, etc would be more scary than a Yorkshire Terrier. Each one of those traditional guard dogs have their pros and cons depending on the person. Just start looking stuff up on the different breeds, I mentioned 3 I bet you can think of more. Then you can come back here with questions we can answer with specific information. No one can tell you what dog is right for you based on what you have said. Good luck.