Quick Bible is a very simple, yet flexible and advanced Bible-reading app. It has a very clean material look, no ads or costs, and yet a host of useful features like multiple verse selections and notes and split-screen reading and color customization.
This is essentially a perfect emulator of a BAII Plus Calculator. Although I have a physical one too, having the same interface available on my phone anywhere is very handy. It's also free with a paid option to remove the minor ads.
The best Hacker News app I've found. It has a lot of features and customization I want and is easy to use. Among these are easy navigation, beautiful material style, and automatic day/night theming. Downsides are that you can't downvote, un-upvote, or flag things from the app, but I've yet to find a HN app where you can do those things, sadly.
This is the best habit tracker. It's easy to use, beautiful, customizable, free, and open-source. You can set whatever habits, reminders, and frequencies you want. You can even set habits for more than once a day or track a quantity with your habit. It also makes beautiful charts of multiple types and support easy backing up and exporting of your data.