Dr. Kenth Svarberg of Stockholm University conducted a study of 164 breeds from 1997 to 2000 that showed the following breeds to be the boldest:
German Shepherd Dog
American Staffordshire Terrier
Parson Russell Terrier (Jack Russell Terrier)
Boldest dogs combine measures of fearlessness, curiosity, sociability, and interest in chasing prey.
Source: The Original Dog Bible, 2nd edition