NewPipe: a YouTube client, as mentioned in the thread, that allows you to play videos in the background, in a small popout over something else, adjust the playback speed, and also download videos for later. Great for lectures.
AdAway: it makes the ads go away by adjusting your hosts file (may require root) so it works both for in-app ads and web ads. I first installed it out of spite when Google failed my LineageOS on SafetyNet and blocked Android Pay even though it wasn't rooted but it made using the system so much more enjoyable I would now root it regardless.
FBReader: a pretty decent ebook reader although the complexity makes it feel like a 2006 Linux app.
Newpipe and Redreader for reddit are my 2 favourite apps on Fdroid. Amaze file manager is also pretty sleek for an Fdroid app. Dns66 is great for adblocking when you are not rooted. Termux is a must have for those who need a terminal. Osm+ is a decent maps option and Mupdf is a fast and minimal reader for pdf files.
Fdroid is great for open source apps especially when you are trying to avoid shady behavior by apps, but since Android itself is full of shady behavior and dark patterns it's at best a bandage on a severely wounded patient.
Slide for reddit is another reddit client. Fully open source under GPLv3, nice looks and hi designed very well such that you never have to reach for keys at the bottom. Wholeheartedly recommended.
The last time i tried Amaze i found the large logo in the sidebar irksome (especially as there was no way to hide the sidebar on a tablet), and it seemed to have trouble generating thumbnails when opening at a directory stuffed with photos or videos (it would generate a few, then stop, then pick up again once you scrolled, skipping 2-3 near where it last stopped etc).
NewPipe is kind of janky, but so much better than the official client. It seems to have the features that youtube wants you to pay for, just by not deliberately crippling the software.
The only thing it's really missing is comments, but some people consider this a feature. (Honestly though, on high quality small channels the comments will often be worth reading, so I do miss it sometimes.)
If you want easy system-wide ad-blocking without root, take a look into Blokada. Also in F-Droid. Implements a vpn through which the traffic is routed, and this way the DNS blacklists can be enforced. Note that, due to android limitations, you can only use only one vpn connection at a time.
AdAway: it makes the ads go away by adjusting your hosts file (may require root) so it works both for in-app ads and web ads. I first installed it out of spite when Google failed my LineageOS on SafetyNet and blocked Android Pay even though it wasn't rooted but it made using the system so much more enjoyable I would now root it regardless.
FBReader: a pretty decent ebook reader although the complexity makes it feel like a 2006 Linux app.