Solid Explorer File Manager is without a doubt, the best file manager for Android devices. And you can get it for dirt cheap for a limited time.
The dual pane file manager app, has a beatiful material design interface (customizable colors) a built in gallery, text editor, FTP client, ZIP and 7ZIP archive creater, which can also open RAR and RAR archives, fingerprint folder lock, cloud file manager, root explorer, and so much more.
You can buy Solid Explorer File Manager for Rs. 10 from the Google Play Store. There are optional plugins available as separate apps, some of which are free and some are paid. But they are only for extra functionality, and completely optional,
It has no ads or push notifications, unlike ES File Explorer which has both and some unwanted stuff like a cleaner. Solid Explorer is devoid of such bloatware, and not only that, I bought the classic version of Solid Explorer back in 2012 (for its full price of about Rs. 100), and it is the only file manager I have used on all of my Android devices since then.