I find it a clunky tool sometimes, but the industry standard for those kind of diagrams in my field is Microsoft Visio. The icon database is fairly high quality

I find it a clunky tool sometimes, but the industry standard for those kind of diagrams in my field is Microsoft Visio. The icon database is fairly high quality, vector graphic, seems to have been done by professional graphic designers. The arrows actually have pleasant curves, it supports theming, etc. I'm sure there are other tools, but the output quality is great. It's expensive though. I've seen some online tools that replicate the functionality and they're awfully close but have even more quirky UI issues that get in the way of getting it done. I don't want to be spending more than an hour on a nice figure for a publication, and even Visio ends up taking me three times that.