Championships. Leadership. Awards. Longevity. Statistical records. Likeability. Talent development.
An NFL  head coach can be evaluated with many criteria. Generally, winning championships over a long period of time is the easiest (or not-so-easiest) way to the top of any ranking. Who does more with less? Who gets his team to the playoffs the most consistently? Who is the best motivator? Whose team is best prepared come crunch time? And who has the shiny hardware to back it up?