I would not pick just one knot. Need to learn a bunch. Even in the Cub Scouts I think we learned square knot, bowline, and hitches. When I was a kid I spent hours with rope and my dad's big Navy knot books - still have them. Wish I remembered just a few more of them at times.

The bowline knot is called the king of knots, and is perhaps one of the most important knots you could learn how to tie. It's used for making a loop at the end of a line. It's very strong, and doesn't diminish the strength of the line much, but is still easy to untie.

In my opinion the bowline. I seem to use in all the time. It did take me a while initially to learn to tie, but after you get it, it's there forever. I agree with "country road" that half hitches are very useful. Knowing how to tie knots properly is a great skill and one you will appreciate throughout your life.

Two half hitches, bowline, and square knot.

The most important knot is the possible one on your head from a fall. It could lead to disorientation, etc.

I like the clove hitch and snares

wow... as I read all these posts, I'm surprised that nobody's mentioned the figure eight--the only knot used for life safety in firefighting!!! as for everyone who said that a bowline won't slip--bull. all it takes is a load shift, and it can slip out. I know lots of knots, but the single one I USE the most is the figure 8 in several variations (on a bight, follow-through, etc). The only down side I can think of is, after being placed under a load, it's pretty tough to untie. Then again, that one knot can be pretty useful to just leave in place.