What external executive assessment resources (assessment tools and/or external firms) do you use for assessment for senior succession planning?

We have had great success using Mitch Salzberg. His email is mitch@salzbergleadership.com

For c-suite roles we use Heidrick & Struggles, Korn Ferry and Russell Reynolds, depending on the position. What I did find is that it is important to first have them focus on the internal candidates and wait for that outcome, this to have good objective evaluation and development feedback given. Depending on how they manage the internal process and if the internal candidate might not be selected and how they treat the internal candidate might give them the search, or not.