As in "BEST" you must mean for the best clubs in the world, because they have the best managers, and assistant managers. The rest all depends on what you're looking for in your team, or even more more importantly WHERE you are playing and with WHAT team. It's a hard question to answer, and like most tactic,staff,player,training questions and threads....it changes from staff,to player, to team, to league, to nation, to personal choice, just like music and ice cream. (but if you "need" help, the higher the rating in their specific choise of job, the better they are at it.)
It's important to have good staff, as it's important to have good players. Without contradicting my earlier post, If you get good physios your players will not get injured as often, and if they do, their recovery time is a lot quicker. Same with coaching specific attributes, like attacking, if your coach is good at coaching attacking, your players will train better and quicker...you must get the drift........Scouts are to be good at 'player potential'...and so on.
You do need good staff, especially your coaches. Obviously the aim is to get 5 stars on each schedule, then your players will improve better/faster. I tend to try to get at least to coaches on each schedule to ease the workload.
i just finish my barcelona 1st season , i notice that barcelona's staff absolutely ****!Even my 1st season with barcelona manage to win La Liga & Champion League , i gain plenty of reputation from it but i still fail to sign better staff from other club .