There is no shortage of file managers on the Play Store. But Solid Explorer remains my favorite, simply because of its Material design and ease of use. For you power users out there, it also boasts drag and drop, FTP/WebDAV/SMB client support, batch file management, and much more. Solid Explorer has just been bumped to version 2.2, and there's a lot to get excited about.
The biggest feature is file encryption. Solid Explorer now allows you to encrypt any given file and folder with AES256 encryption. It decrypts files transparently to the user, so you don't have to worry about re-encrypting them when you're finished. You can also use your fingerprint to unlock files (as well as to login to cloud storage).
There's other useful additions here as well, such as multi-window support on Android 7.0, automatic day/night themes, homescreen shortcuts, and more. If you want to try Solid Explorer for the first time, there is a 14-day trial available.