What are your must-have programs?

A good kit IMO, in order of priority:

• Cherrytree; nominally for making hierarchical lists but you can basically use it as a wiki for your entire life. You can theme it yourself too, if you think it looks too retro out of the box.
• Syncthing, for keeping files synced between devices without having to use a server.
• Qbittorrent, for getting files you need. Remember to install search plugins.
• KeepassXC. Password manager (local, not on a server, use in combination with Syncthing).
• Convertall, for unit conversions.
• Calibre, for managing an ebook library, converting formats, removing DRM, transferring to ereader etc.
• Rhythmbox, for music library, podcasts, internet radio.
• Shotwell, for photo and video library. Easy to use, supports tags (metadata written to image files).
• GIMP, for image manipulation. It's extremely versatile, comprehensive and versatile. 3.0 is due out soon and will include non-destructive layer effects. Heavyweight piece of software, so expect a learning curve.
• Ardour, for music production. Heavyweight, steep learning curve.
• Flowblade for video production. Lightweight, easy to learn.
• Libreoffice, desktop publishing.
• Librewolf; privacy-focussed web browser.
• Thunderbird; highly organisable email client.
• Freetube, for watching youtube videos without all the ads and tracking. Local subscriptions and playlists, which you can export to use with Newpipe on Android. Also lets you download video and audio.

If you like the terminal also add:

• ranger; file manager
• newsboat; RSS feed reader
• yt-dl; download videos from youtube and many other sites ;)
• w3m; command line web browser. I like to use this in combination with newsboat.

Enjoy!