Ok I had a look at some recent Everton managers' win percentages with Everton (help me god!)
Silva - 41.2
Koeman - 41.38
Martinez - 42.7
Moyes - 42.08
Smith - 31.55
Speaks to a very bad time under Smith, followed by much the same from each of the following managers, with Martinez slightly in the lead.
Some famous successful Everton managers;
Kendall - 54.1 / 37.7 / 26.2
Catterick - 46.6
Sam Allardyce - 38.5
Top bin;
Pep G career win % - 72.9
Candidates;
Howe - 43.2
Wilder - 44.4
Benitez - 49.8
There you have it, a long chain of OKAY. Can it improve? A bit of time will tell, otherwise the only candidate with any pedigree above the recent lot is Benitez