First and foremost and imo most important, is transparency. A breeder must be honest with me, even if its not the pretty or nice side of things. They need to be willing to discuss things and be open to criticism or really tough questions.
Secondly they need to have high standards for their breeding stock, even if their priorities are not the same as mine, they need to at least HAVE them.
Health testing and titling is negotiable, and doesnt feature on my list, oddly enough.
Everything else will vary by breed and are nitpicky things like must not use XYZ bloodlines, or whatnot.