Tasker. Basically an interface for writing scripts for your phone. Even if you don't have a use case in the beginning you'll start finding things to do with it.
It's just a really great alarm clock app, but with tons of other sleep tracking functionality. I've always had trouble sleeping through my alarms, but I never do with this.
I find recipes online, download them to the app stripped of all the online recipe bloat. It sorts all the information automatically, including notes and nutritional info. I can check off ingredients and highlight directions, edit tags, compile menus, add my own notes and write my own recipes, it automatically provides a grocery checklist, has a serving calculator to adjust amounts for whole recipes, built in timers, and that's just the basics off the top of my head.
It's free up to a certain amount of storage but I think all the features are available.
I love Simple Audiobook Player+. The UI is super minimal (and really maxes out the whole OLED black thing if you choose it) without compromising on features that are kind of essential for audiobooks (e.g. delayed pause/sleep timers, speed settings, volume boosting, an EQ). My favorite thing is the "undo seek" button. I'm an oaf who is constantly inputting accidental touches. When I was using Audible, I'd have to manually find where I was after accidentally hitting the next chapter button or moving the dot on the progress bar. SABP lets me just undo that shit.
It hasn't been updated in a while, but it doesn't need updating when it does its job so well. There are no ads, no marketing notifications, just books. It's like a program from coreutils in app form. It might be a bit ugly or outdated looking, but I'm about that.
Read Era is technically free, but I paid for premium years ago and have never regretted it. I can open any kind of uncorrupted book file, from the Amazon reader format to PDF to epub, and everything else I've ever come across. It has a great search function, and the ability to file a book into a custom 'Collection'. You can edit the details of a book, like adding Author or pusblisher info, add your own personal notes to a page or highlighted quote, see an aggregate of all your highlights in a particular file, and adjust the font, background color, and contrast to your hearts content.
I make my whole family use it now, cause I love it so much and Premium works on Family share.
just scrolling trough my phone, here's some I like
app opps - lets you change permissions for apps, handy if you want to have multiple things playing audio or use google photos without it scanning your phone.
calcu - it's a calculator!
simple draw - exactly what it says on the tin
es file explorer pro - versatile, but not bloated
moon+ reader pro - a handy reader for all sorts of docs, including search etc
polarr - a photo editing app with features not a lot of apps have. The devs are pushing some dumb filters tho
handy photo - same as polarr
mx player pro - got videos to play?
poweramp - a music player
poweramp equalizer - is what it says on the tin
sd maid pro - for clearing out old files and such
poweraudio plus - used to be the only app with a parametric equalizer. Now poweramp does too
ultrachron - just a nice timer/stopwatch
unified remote - a remote control app, be careful tho, I doubt this thing is secure
web video caster - also downloads videos from plenty of places
and a couple more apps, where the developer has decided to pull the lifetime license and move to a subscription, even after I had bought it
Cryptomator is a fantastic way to securely upload your stuff to cloud storage providers like Google Drive, OneDrive, etc. In my case, I use it to have an encrypted blob of my stuff with me on a drive when I'm out and about.
They also give you the ability to purchase a license independent of Google Play if you didn't want Google to get a cut.
HiPER Calc Pro. A great scientific calculator I use constantly. (There is also a unpaid, ad-supported version, and the ads weren't too intrusive the last time I tried it)
Otherwise, I usually prefer free open source solutions (FDroid), but I regularly donate to keep the projects alive.
1 and 2 are small dev studios that I am happy to support.
A file explorer allowing for me to transfer files over the network. When Solid Explorer suddenly didn't seem to want to do network transfers anymore (likely because Windows updated something), I waited for that app to update to fix the issue. It never did. I found that MiXplorer was a good alternative that transfers files over the network just fine and works nice and fast as well. The interface takes a bit to get used to (meaning it isn't the same as Solid Explorer) but the app is certainly worth using. Importantly, I can transfer files over the network without issue again.
Notably, this app is free to download (from XDA) however the Google Play version is not free. The Google Play version (which supports development) is a one time paid fee.
If you're a fan of customization and getting your home screen just how you like it, KLWP and Nova Prime.
Nova Prime is a launcher with a bunch of customization options for how your app drawer works, how apps are laid out, how big they are, how folders look, etc. I use it for the fact that i can set folders to have the icon of the first app inside, and a gesture so that tapping them opens that app, while swiping up opens the folder.
I know the Windows Phone experiment failed but it was my first smartphone that I bought and not just inherited my dad's work-iPhone when it became deprecated. I really love the "live tile" type home screen and Square Home improves on it as well, instead of just carbon copying it.
Other than that, the FUTO keyboard / voice input and Grayjay, although these three technically offer a lifelong free testing period, similar to WinRAR, but even less obnoxious because they don't even remind you that they want you to pay for them.
CamScanner (intuitive and powerful scanner app that processes images exceptionally well and interfaces cleanly with all sorts of other apps) and Hiper Calc Pro (scientific calculator that shows you your input and looks like a classic calculator interface)
Tasks.org is a wonderful open-source todo/task app, that has a low-cost monthly subscription to use it's syncing ability. It's worth it to support FOSS wherever we can.