The short answer is that a harpist with the ability to teach makes the best teacher. That teacher must be an advanced harpist him/herself. But holding an orchestra position is no guarantee that that person can teach. Unfortunately, most conservatories and schools will want teachers on their faculty who have a resume that looks good in the catalogue, and frankly, they don’t really care if that person can teach, or turns out quality students over the years. This may sound harsh but it’s true.
Being an orchestra player(on any instrument) is very different from being a soloist on the same instrument. Soloists are frequently poor orchestra players, and orchestra players are frequently poor soloists for this reason. And being a first rate orchestra player or first rate soloist does not gaurantee a great teacher.
A teacher may be great for one student and not for another. I think in the end each student has to seek out the teacher who can provide what that student needs.