I'll go with John MacLean from recent history. Tougher to judge guys from the early 80's when the team was so brutal to begin with.
It's definitely MacLean. Lemaire came in after him and turned the team completely around and they played great. They were 9-22-2 under MacLean which is a 49 pt pace and the same that the Avs are currently on. Under Lemaire they were 29-17-3 which is a 102 pt pace. More than doubled their pace after they canned MacLean