It's tough to pick just one,,,,,, My preferred technique now is mulching in fall with aged manure topped with wood chips. I guess if I could only have one I would say just wood chips. They can be applied any time of year and the benefits are many.
I like to have multiple amendments to work with. We've come to a point where we use a combination of vermicompost with biochar, aged manure, wood chips, and yes... store bought liquid and granular organic fertilizers( I hear you booing). It's useful to have several things in the toolbox.
But if I have one vote, wood chips.