Noto Sans for sans-serif text (and the OS)
It’s legible, standard-looking and support about every writing system in the world. You can install it on Debian using # apt install fonts-noto, some others like -cjk and -extra help with the “supports about every writing system in the world”-aspect.

Merriweather for the serif font fallback for the browser, as well as TTRPG campaign printouts
It’s very legible, and looks quite sexy for a serif font. There’s no package for it currently (although AUR and Nix users might have better luck), it has to be downloaded from Google Fonts

JetBrains Mono for the terminal TUI’s
It looks a bit playful, like lego-letters, is legible and supports about every writing system in the world. # apt install fonts-jetbrains-mono.

Although I use…

Verdana for source code
It differentiates every character well and leaves enough space to easily recognise special characters such as brackets. And I don’t believe monospace fonts are more legible. It’s included in ttf-mscorefonts-installer but the font is not open-source.