Depends on the diagram. But a lot of times it's OpenOffice Draw[1]. I might also use Archi[2] or GraphViz[3] depending on what I'm trying to do. I've also dabbled with using Papyrus[4] but it hasn't yet become a routine part of my workflow. Maybe it should though...

[1]: https://www.openoffice.org/product/draw.html

[2]: https://www.archimatetool.com/

[3]: https://graphviz.org

[4]: https://eclipse.dev/papyrus/