Well, since you're asking about the most important principle, I'd bring it back to the core ethical points, which virtually all permaculturists agree upon:
- Earth Care (caring for the living systems in this world)
- People Care (caring for the humans living in, with, and off this world)
- Fair Share (generating surplus and reverting it into the care of the earth and the people).
Just to clarify: this is permaculture, which includes horticulture (fruit trees and orchards) but also so much more than that.