That's a very worthwhile one to learn. I'm pretty sure the website mentioned has it. I've seen that site before, and I seem to remember that's where I first learned how to tie the Double Dragon. It should be categorized under "single loop knots". Easy to tie and untie, very strong and secure, has a good "lead" (i.e., the loop comes off the standing part of the rope nice and straight, not cocked off at some angle). It is also easy to "inspect" (i.e., look at the final product and determine if it's properly tied or not).

The only very minimal negative to this knot is that it takes slightly more time to set it. All knots should be "set" before use (i.e., all slack taken up and things tightened so that wrapping turns lie nicely snuggled up and adjacent to each other). Because there are two wrapping turns (hence the "double" in the knot's name) it takes a second or two more to tighten both of these, as opposed to tightening only one wrapping turn found on many other loop knots.

One knot you didn't list in your "known knots" is the "constrictor". This is a definite choice for anyone's "I should learn this" list. Also learn how to "slip" it (i.e., finish the knot by tucking a loop, not just the single running end of the rope). Easier done than explained. The constrictor is so good that it can be considered a "permanent" binding, unless you slip it.