SimpleMind is a good mind mapping software. An example of a mind map created using SimpleMind is given below.

Mind map

SimpleMind has apps for macOS and iOS. iOS app can sync in local networks with macOS app without having to sync with a third-party cloud service. This does not work with Android app however. The UI is smooth and the mind maps produced are even more nicer. The themes, customization are also rich enough that I don't miss anything that a mind mapping software requires.

The .smmx is a proprietary file format for storing SimpleMind mind maps. We can export in other formats like OPML, but it will lose any rich content information, in case one needs to use another app.