When I need to use the most intensive black possible on a certain paper (medium) without exceeding the specified ink coverage, I create an RGB black = 0 and convert it to the CMYK color space of the paper. This ensures that I will not exceed the ink coverage.

If your question is not specifically limited to the most intensive black possible on a certain medium without exceeding the ink coverage, but dark colors in general, then your question is like asking "what is your favorite color  in general (not just blacks).

Hello Bill

Yes, it's an informal term. CMYK black is a dark blackish grey, as you know, and the term "dense" just means turning the dull flat cmyk black into an interesting version of black. A general purpose spec might be simply 100k overprinting 70c. In the old days this was known as putting a "shiner" tint under the black. Or if you wanted your black to have a reddish tinge, you might chuck in a blend of magenta and yellow instead of cyan and so on. So you can make up your own "dense" black to suit whatever illustration colour bias you are working on. That's just rewording what JET has already said. Personally I avoid rich black like the plague. I like my blacks to look and output exactly as they are intended.

One potential caveat to using rich black for art printed on a physical offset printing press: trapping. Registration, although "theoretically perfect," is never "actually perfect." So pull the CMY colors back away from the edge of the black (at the printer's recommended trap amount), to avoid some of them peeking out from around the edge. In this case, 100% black is best, as opposed to the 90% shown in emil emil's example.