I always assumed that the best silicon was sold as K series CPUs. They are the ones that get released first. CPUs with defective Intel GPUs get set aside and are sold later as KF CPUs. The ones that cannot run at maximum speed without needing an excessive amount of voltage are set aside and are sold off later as non K CPUs. This is the standard procedure that Intel has been using since the beginning of time. It allows Intel to sell off as much of their production as possible. They hate to let any production go to waste if they can sell it off as a lower binned part.
@tcprix
I would always recommend a K series CPU, even if you do not plan to overclock it. They tend to be the highest quality.