If You Only Learn To Tie One Knot...

...Make it the figure eight knot. This is a very versatile and relatively strong knot that's easy to learn and execute. With just a little variation, it can function as a stopper, a bend, a hitch and a loop.

The basic form of the figure eight knot (shown above) is meant to act as a stopper, to keep a rope from running out of another knot or a retaining device. It's superior to an overhand knot in that it doesn't bind under stress and can be untied when it's no longer needed. An overhand knot, on the other hand, will often be locked tight, requiring it to be cut off, rather than untied. The figure eight knot also retains more of the rope's breaking strength (roughly 75%), compared to the overhand knot, which nearly halves the strength of the rope.