All WordPress developers need to have excellent skills in PHP, Java, JavaScript, CMS development, HTML, CSS, Basic Photoshop, and Bootstrap.
Top WordPress developers are those who have skills beyond the basic and generally preferred categories. Examples include WordPress developers with extensive experience in Python and MySQL or WordPress programmers with skills in Mobile UX and UI design and web design.
If your company specializes in web development, it would be great to find developers with skills in WordPress as well as tech stacks like MEAN or MERN. Such a combination of skill sets helps you take on many projects using the same core team.