Like everything else, judgement of "the best" WNBA coaches is problematic.
Reeve is one of the best, Brondello better than folks act. Thibault (Mike) was very successful.
Of the much more historic group, clearly Van Chancellor, Bill Laimbeer, Brian Agler and Michael Cooper stand out.
Hammon and White and a number of other coaches "look" very good, but don't have the lengthy record to make a case. Even the successful coaches in the early days just didn't stay all that long, as a rule. I do expect Hammon and White to eventually establish themselves among the best, given time.