Slightly off topic but for me a good terrier is one that has a great nose, hunts through cover like a hound, has a good voice which it only uses when pushing its quarry through deep cover (handy for tracking it), is rock steady with livestock, knows how to sit still and keep quiet when told to and no1 marks so good you would put your house on it. But then i hunt rabbits.
atb
Jai.
edited to add : It's also got to be small and well built.