I would’ve said Infinity for… something, but we know how that turned out. So, I’d say it ir most likely Fennec F-Droid, it is the first app I install in every device that isn’t a tablet
OsmAnd GPS map and route planner.
No bullshit, and it works offline without ever bugging you to download new maps. Great route planner and directions. Simply amazing IMO.
However this is #2 after Firefox, but Firefox is already prominently mentioned.
Jellyfin! I love having access to my own movies and audio audiobooks wherever I go!
Firefox by a huge margin, because it’s integrated into how I use the internet so well now I don’t even notice it’s a FOSS app.
As far as more niche apps goes, Joplin. Since they added WYSIWYG editing it’s an easy Evernote replacement that just does what I want in a note taker/web clipper app.