The most important are aircraft maintenance. This is from a pilot's perspective.
The least important, Upper Mgmt. Not all of them, but some go from airline to airline, stuffing millions, if not tens of millions of scarce dollars (some via 'free' stock: this is in their contracts) into their pockets, and lacking people skills, they continue to damage or cripple airline after airline, from a long-term perspective.
Check what dangerous situations happened as a result of their type$ of "outsourcing". Many of us anticipated a major incident or crash this winter-at our company. Events have taken place which we have never heard of before. Never.