Not sure there's too much competition for that honour, but at the moment it has to be...

Graham Potter of Ostersunds in Sweden.

They were a permanent fixture of the bottom tier in Sweden when he took over, but since then they have seen a meteoric rise. They made their way to the top division, won a cup and qualified for the Europa league.
They got through the qualifying rounds and have been very impressive in the group stages. They won away in a very tricky tie in Ukraine, and last night earned a 2-2 draw with Athletic Bilbao to top the group and look like they are in a very good position to make the next stage.
Sooner or later someone at championship level or above will make an offer for this guy.