Depends on the skill set of the head coach, imho.
The immediate answer is for them to be good recruiters and they need to be because they spend so much time developing relationships with kids but they basically have to be what the head coach isn't. That sounds weird, but a staff needs different skill sets to be effective.
If the head coach is more of a recruiter, then the assistants may need to be more of a strategist to help the head coach in game situations/game plans, etc. If the head coach is a better bench coach than recruiter, then the assistants better be good recruiters. They also need to be good skills teachers, aka shooting to help develop players.
I think they have to be diverse in their strengths to make a good staff.