In the two playoff games so far, Al's on/off differential per 100 possessions is a whopping +32.1, Brogdon is +24.2, Hauser is +10.7, Tatum is +4.7, and Smart is +1.8. No one else is positive. In the Regular season the positive players were White 9.8, Tatum 5.5, Rob 4.5, Al 2.5, Blake 2, Hauser 1.6, and Muscala 1.5.
So between the regular and post season the only people that were positive in the regular season and are in the postseason rotation and have maintained the positive thus far in the post season are Tatum, Al, and Hauser.
So not counting Tatum and Brown, that would lead to Al being the 3rd most important counting both the regular or post season or pretty clearly White if you are looking at the regular season only.
As for the best, that is probably Brogdon since he statistically has been the 3rd best player for the team over the course of the season.