As a rule ATPL with 10+ years on heavies, or a Chartered Engineer with a fairly wide and varied experience of aircraft operations.
They are also particularly fond of engineers who fly, or pilots who have taken the trouble to understand engineering properly.
Everybody needs extremely highly developed report-writing and analytical skills.
That and diplomatic skills at-least on the Kofi Anan scale (although some might argue that this latter has gone by the board a little in recent years).
G