I am not an engineer so I cannot attest to what is the most challenging technically, but I can tell you what I believe to be the most challenging specialization that I have come across in relation to conducting an executive search, and that would be for geotechnical tunneling engineers.  I can find the names, but there are only a handful in each major city, which makes it very difficult to not only find someone for my client locally, but to find someone with that experience who is open to relocation of course is even harder.