Linux in general. MacOS if fine, but the app ecosystem is often annoying. And Windows is just a complete dumpster fire these days.

I was setting up a Plex server, but when I noticed I had to pay to be able to play my own content on my phone I immediately switched to jellyfin. Haven't been able to test it yet, but as long as I don't need to pay them to be able to watch my own content on my own devices on my own network, I'll be happy!

(neo)vim

I would love to use Jellyfin but it (indexing, changing metadata...) is too slow with a few hundred movies and shows on my Synology. Plex is way faster.

On Android; mpv, KeePassDX, FlorisBoard, AntennaPod, Read You, NewPipe, Jerboa, Unitto Calculator, CloudStream, Aegis, TrailSense, OpenKeychain, K-9 Mail, EDS lite, ViMusic, InnerTune, GrapheneOS Camera, Librera FD ...are my favourites.

I could be biased but 2009scape. While originally a Runescape clone of 2009, they've preserved the integrity of the game much better than the official versions

I switched to Jellyfin two years ago and never looked back.

Pandoc. I'm not even sure there is a decent alternative.

Kicad beat the crap out of EagleCAD. So much so that autocad just folded and discontinued EagleCAD.

I use Gimp and Krita, ShareX also with the complementary Extension, integrated with FileCoffee, old but gold VLC media player, PicView image viewer/editor (IMO best alternative to IrfanView), ProtonVPN (yes, it's OpenSource), Crow Translate, FreeTube, Portmaster, Cherry Tree editor, apart of some games (The Dark Mod, Armagetron Advanced, Scorched 3D, and some more)

The scanner app on Linux is far better than Windows: auto preview white scanning, auto pdf creation WITH multiple pages.

Moodle is better than Google Classroom

Pidgin. Consume soo little memory in comparison to other chat applications. Is really fast, just try start and scroll history in a chat conversation. But it looks ugly.