• Pdf Viewer Plus - simple PDF viewer
• quickDic - offline dictionary
• App Manager - full-featured package manager and viewer
• Librera PRO - All-Format eBook Reader
• K-9 Mail - Full-featured email client
• obsqr - fastest and most lightweight qr code reader
• Kernel Adiutor - Manage kernel parameters
• Metro - Best material design music player

• Mull: web browser
• QKSMS: text messaging with import/export support.
• K-9 Mail
• andOTP
• Vanilla Music
• DroidShows: because I sometimes forget where I left off when watching a show
• OsmAnd~: it's so-so for road navigation, but absolutely fantastic for hiking
• and finally lemmur, naturally

• Aegis Authenticator, IMO the best TOTP app out there.
• mpv-android to play almost any kind of multimedia files.
• Organic Maps for an OpenStreetMaps-based navigation, and doesn't require an Internet connection to operate.
• StreetComplete to fill missing OpenStreetMaps info you encounter, in an almost fun manner.
• Syncthing-Fork for a Syncthing client with some good mobile-specific tweaks.
• Termux to emulate and run a bunch of console-based softwares (like youtube-dl, etc).
• AirGuard to detect if you're being tracked by an Apple AirTag without your knowledge.

Just recently found Barista. Its an alternative instagram client that lets you browse privately and does not expose you to their stupid algorithm. Also Infinity to go on Reddit.