For drawing scientific illustrations, engineering diagrams, and block/process diagrams which are the best softwares for producing those at high images resolution.
Inkscape is a good, free software , and it's gaining popularity among academicians. AND of course MS Visio, which is-in my opinion- easier to use; besides it allow saving files in almost all the common formats:PNG, bmp, PDF... tc
Microsoft Visio which is a graphics application and is part of the Microsoft Office family, Simplify complex information through simple, easy-to-understand diagrams.
Draw.io (free and open source) - straightforward, allows saving straight to Google Drive or Dropbox. Also has a Desktop version.
Gliffy (free up to 5 public diagrams, then pay-for) - clean layout, has a 'Confluence' plugin for wikis
For MAC/IOS, Omnigraffle is popular for that sort of thing, if what you want is to make connected-block arrow-heavy flow diagrams quickly.
Dia is a free open source diagramming tool and available for Windows, Mac OSX and Linux. Its been around for years and is popular with both software and electronic engineers.
yEd is also good at this, is free and has Windows, Mac and Linux versions.
I recommend Mind the Graph (www.mindthegraph.com) to scientific illustrations. it is an online platform, to create graphical abstracts, presentations and infographics. There are thousands of illustrations available and you can request new if you don't find what you need.