"It will not be necessary endlessly to make the point that readings which we judge to be superior taken by themselves are only to be preferred to others if they are supported by at least a few ancient witnesses. I pay no attention to readings which are supported by no adequate witness, but only by late and worthless ones. However, the more internal indications of its excellence a reading displays, the fewer the witnesses necessary to establish it. In practice, therefore, it can happen that a reading exhibits so many and such obvious indications of its worth that two witnesses, provided that they belong to different types and families, or even a single witness, are enough to support it."
This seems sensible in general, don't you think?
What are the most compelling readings supported by only a single witness?