The tools mentioned above should only be used for downloading free online videos for personal use. Different websites have different policies and it is recommended that you review these policies carefully and proceed at your own risk. Fossbytes in no way encourages the illegal distribution or duplication of copyrighted content. Before downloading, copying or distributing any video content online make sure you are not violating the law.
Totally free, 5KPlayer from DearMob is a media-playing utility that is a lot more than a downloader—but it's got a heck of a downloader integrated. Promising no viruses, ads, or plug-in requirements is a good start. It is, sadly, one of the few I tested that asks for a registration of your name and email—you have to do that to get the full download function across 300+ sites. You may still use it if you don't register; I didn't and was able to download 4K vids from YouTube.
That said, with Android it is possible to install apps without going through Google. The latest is InsTube – Free Video and Music Downloader just for Android—you can only get it at InsTube's site. You download the APK (Android application package) file to sideload on an Android device. Find it in your device's downloads and click it to install. (You may have to go into your security settings and enable "Unknown sources.") There is a similar side-load app from YTD.
When you’re looking for something to watch or listen to you want something that you’re going to enjoy, but you also want a good amount of value. With downloading an entire YouTube channel, you’re not going to have to worry about that because you’ll be able to check back in on a number of different videos and different episodes that you may or may not have seen before. This also gives you a whole lot more freedom to check out something different without having to worry about downloading yet another episode or video that you don’t have the data for.