I used to make a lot of use of Newpipe, and probably should get back to using it more, but I've admittedly been preferring to use the mobile site with adblock for a while now, only switching to NewPipe for video downloads, due to how often NewPipe was having issues (presumably caused by frequent Youtube updates). Is that better these days? I sure hope so, it's a lovely app otherwise that I'd love to use more.
As for my own favorite, it's hard for me to pick, because there's several I make use of daily. In no particular order:
Simple Keyboard - Has been my main mobile keyboard for years now.
Editor - Just a simple text editor, comes up surprisingly useful.
Diary - I personally use this as a dream journal, and it works well.
Simple Calculator - Does exactly what it says on the tin. For some reason, my new phone I got last year doesn't even come with a calculator, lol.
Simple Calendar; Simple Gallery - I prefer these to the default Android apps.
Simple Voice Recorder - I make music sometimes, so this is useful to hum a quick tune before it leaves my head.
TimeR Machine - I use this during daily exercise as a HIIT Timer.
I'd also recommend RedReader, but I can't in good faith do that anymore because of what's going on with Reddit these days. Haven't used it or the site much since June.
I'd say my favorite is probably Simple Keyboard just due to how often I use it (read: literally all the time, because it's a keyboard), but I've gotta give mad props to TimeR Machine. It's genuinely a really polished app, and even gives you a quick walkthrough on how to use it on first startup that's pretty helpful. It'd be neat to see more apps with those options.-