It must be the bowline knot. It’s not known as the King of Knots for nothing. (yes, that is a legitimate title. Trust me bro.)
The bowline’s name (pronounced “bow lin”) originates from seafaring days, but it’s been around much longer – for thousands of years – as it’s simple-to-tie, secure, and easy to untie even after loading.
As it’s easy to untie, it works best when under tension. It can work itself undone when not under a load. There are more secure versions of the bowline, like the double bowline, which are derived from the original knot.
Since the bowline knot makes a fixed loop at the end of the tope, it’s particularly useful in the outdoors when you need to tie a rope against a tree. You can then attach another bowline knot to the end of that knot, extending the rope.
You can learn this knot in 10 seconds, and with a few reps of practice, never forget how to tie it. It’s an essential survival skill to commit to memory.
