Yeah my top 3 is websequencediagrams, PowerPoint and draw.io All depends on the information I'm trying to convey and the formality, Truthfully, PowerPoint is my goto for explaining flow and quick 'this speaks to that' style network stuff.

I also get a bit of millage from time to time out of the Graphviz online demos. I use those to throw together graph-ish stuff on the fly.