I hate "this!" comments, but F-Droid is the thing that keeps me an Android user and makes me a die-hard proponent of alternative app stores (and why I'd use an iPhone if I could get something like F-Droid (with Termux) on it). Even phones with no other app store can install F-Droid, and it has enough good software that I can do most everything I need with only F-Droid.

    - FFUpdater for installing browsers (Mull on my phone, Brave on my tablet because keyboard support)
    - Termux for all my CLI work
    - KeepassDX for passwords
    - Syncthing for syncing
    - Etar for calendaring
    - VLC for media
    - Kore to control my media center
    - RCX for running local webdav servers (good for hosting Tiddlywiki and FeatherWiki locally on-device with WebDAV-based saving)
    - Tiddloid for notetaking when I'm not using RCX instead
    - Conversations for XMPP chat with family/friends
    - KOReader for ebooks
    - MullvadVPN for ... VPN
    - OsmAnd~ for mapping, including offline maps
    - "Puzzles" and Unciv for gaming
    - Trail Sense for navigation/survival

I know this is a laundry list, but my point is that it's a trove of exactly the sort software I like to develop and use. Thanks for mentioning it!