Ranger is by far the most useful file manager I have ever used. It's quick and powerful. The only thing that keeps it in the minor leagues is how big the learning curve is for anyone that doesn't know Vim shortcuts.
I'd argue such a file manager is meant for people who prefer and are used to Vim shortcuts. I just tried Ranger. Got image preview and Solarized theme working, but it does hang on previews it seems.
For a GUI one, I can recommend fman (fman.io) as it uses the Sublime paradigm (shortcuts plus a command palette). It isn't FOSS though.
Also worth mentioning nnn, fff, and filet. All Ranger-inspired file managers that are faster and more lightweight (listed in order of ascending speed).
