It depends on the exact problem and how long term you need to documentation to exist. For a lot of things physically drawing out diagrams is great. It's fast to do and based upon the questions you get it's easy to draw more detail on the area being discussed.

For more long form docs I tend to use a mix of LaTeX's TiKZ (though I used to use PSTricks) for block diagrams and for a somewhat faster workflow I've used some of the diagramming tools which integrate nicely into an asciidoc based document. That can be graphviz, blockdiag, seqdiag, etc ( https://asciidoctor.org/docs/asciidoctor-diagram/ has a decent list of some of the options).

With any option it's easier to quickly sketch something out than it is to have some clear well organized diagrams which don't need any external context to understand.