My vote for most useful knot is the figure-8. Can be used to secure something to a fixed anchor, as a bend, or to put a loop in the middle of a line (something

My vote for most useful knot is the figure-8. Can be used to secure something to a fixed anchor, as a bend, or to put a loop in the middle of a line (something like an Alpine Butterfly is a better option there, but if you only know the figure-8, it will get the job done).