Re: Essential knots?
If you know a few knots, there is no reason not to know many more knots that are simple variations or combinations.
If you know the figure-eight, then it seems easy to know the figure-eight loop, figure-eight bend, double figure-eight-loop and the one-way knot.
The bunt-line is a clove-hitch over the same line.
Double the "rabbit-hole" and you have a double-boline; triple it and you have a triple-boline; make the "rabbit hole" a clove hitch and you have a water-boline. The boline is not all that reliable when not under tension -- finish it off with a half-hitch or feed the bitter end back up through the "rabbit hole" for a Yosemite-boline.
As mentioned, the Alpine butterfly is also a bend. Also as mentioned surgeons-knot is a square-knot with a an extra turn and more secure.