Forgot to add what I believe makes a good breeder - but in all honesty, I, like several others in this thread, am relatively ambiguous as far as requirements go. I do not have a checklist or many specifics that each breeder must meet for me to decide they are "reputable". Often my answer is just "it depends" if you were to talk to me about a breeder and ask me my opinion.
For example, I am going between two or three breeders right now. Of those three, two of those breeders have not title all, or probably even most, of their current breeding dogs. And these are German Shepherd breeders, not toy dog breeders!
Not toy dog breeders... and not Schutzhund breeders, either, nor are they agility breeders, flyball breeders, show breeders, performance breeders, pet breeders, etc. They are German Shepherd Dog breeders, because when I get my next dog, I want a good German Shepherd Dog. A Schutzhund 3 title does not tell me anything that the breeders I am considering cannot, even if it is about the breeders' own dogs. At least, it doesn't tell me anything that I care very much about when I look for a dog and for a good breeder...
I often find that people put a lot of faith into titles when it comes to declaring how well a dog represents his/her breed, and in deciding the dog's own breeding worth and the contribution the dog can make to the breed. I don't believe that any title is a better judge of the dogs I hope to produce my future pup than their breeders, because I put a lot of trust in these breeders' knowledge and judgment and years and years of experience. Is it a risk that I am taking? Yes, probably. But not any more than if I were to look simply at titles and other such forms of credentials. I am not looking for a Schutzhund dog or an agility dog, but a German Shepherd Dog. A Belgian Malinois is a favorite among the Schutzhund people and many are easily titled to a SchH3. But that doesn't make them good representatives of the German Shepherd breed, obviously!
Now, am I putting down breeders who DO title their dogs? Not at all, and actually, I prefer dogs to be titled as opposed to untitled. But if a breeder I like does not title their dogs, will that lower my opinion of them? Not in the slightest. I know what I want and what I like to see. My German Shepherd's own breeder (and his daughter) are DVG Judges, and Trent's sire is IPO3 VPG3 and dam SCHH1 (and KKL1). Titles are good to have, but when it comes to making my own decisions about the type of dog I personally want, I do not care if the dogs are titled or not.