A topic close to my heart! One thing though, lots of these tools are "offline", but some are found here as a repackaged service online. Personally, if I'm diagramming some important things, I don't want to trust yet another third party, and largely prefer "offline" tools, and I suspect there are others that have the same view. For testing the tools this is great though.
Blockdiag [3] is the one I tend to use the most, with both Mermaid [2] and the unoriginally named "diagrams" [1] being the two others I find the most interesting.
I have enjoyed using PlanUML, but getting a local renderer for team use is a big lift. I switched to Mermaid-JS last year. Using mermaid, we can do a number of diagrams in Markdown code blocks. VSCode has a markdown preview enhanced that can render these, and Gitlab will render mermaid. Also, you can get a mermaid plugin for Confluence.
It may not be the "best", but it's exceedingly versatile and has a low barrier to entry, which is a huge win for us.
I have troubles with Mermaid. While I love what it does, the SVG it generates can draw out of bounds and some of the chart is not visible. I haven't tried some transform hack yet but it has some issues.
Blockdiag [3] is the one I tend to use the most, with both Mermaid [2] and the unoriginally named "diagrams" [1] being the two others I find the most interesting.
1. https://pypi.org/project/diagrams/
2. https://mermaid-js.github.io/mermaid/
3. http://blockdiag.com/en/