This Can Save Your Life - The Bowline Knot
Called "the king of knots" by many, the bowline knot is an ancient maritime knot that can be utilized so many ways. It's a very reliable fixed-loop knot, renowned for its strength, relative speed & ease of tying (even one-handed), ease of untying, and because the bitter end can be passed around an object as it is being tied, making it quite common in mooring ships and aircraft, and is a very common rescue knot when securing or extracting personnel or equipment.
Most people learn this with the aid of a mnemonic - There's a rabbit hole around the tree, up comes the rabbit through the hole, it runs around the tree, and back down the hole. A common mistake is square-knotting the "rabbit hole," which results in a weak sideways bowline that tends to slip. With a little practice, you can do it with your eyes closed, without the mnemonic.
Once you have that down, there are several variants you can learn, that have their own special applications and benefits. The double-bowline adds an extra loop to the "rabbit hole," making it more resistant to loosening when tension is removed. Wet rope has a much greater tendency to slip when knotted, so adding a clove hitch before passing the working end though makes the water bowline more effective around water.
One variant I'm most familiar with are most commonly used for hauling heavy loads, climbing and rescue. The first, is the Yosemite bowline, which is more-secure than the standard bowline, easier to undo after a heavy load, and easier to verify before climbing due to a distinct figure-8 shape. After tying a bowline, before pulling it tight, take the working end back around the loop in the same direction of the original turn, and then pass it back up through the "rabbit hole."
I'm also a big proponent of learning the one-handed bowline, as it could really save your bacon when things go sideways. The biggest advantage is that you can tie this knot around your waist, while holding onto the rope, in the event you need to secure yourself while holding onto something important. I can never seem to describe this, so instead I'll link to the masterminds at ITS Tactical, who have a great step-by step video.
The bowline knot, and its variants, join the figure-eight knot, reef knot and clove hitch as vital knots for just about anyone who spends their time on the water, in the wild or having to tie anything down or secure something. I can tie a bowline in the dark, one handed with either hand, sideways, upside down, and with all sorts of cordage of varying width. I consider it among the most vital skills I've learned over the years, as it's just so versatile across so many fields.