Linux

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.

Biolinum O for desktop

Liberation Mono for terminal

Inter everywhere

SegoeUI, it’s damn good and well made

Personally, whatever is default.

I know that may sound weird, but I'm a huge fan of sane defaults that I don't even notice are there.

Poppins, RobotoMono, Comfortaa and OpenDyslexic

Pusab (I'm a gd player)

Whatever the default font is

VictorMono, has a cool cursive, mono spaced font.

I don't have a favorite system font, am I meant to? I did try to play with fonts at one point but the process of finding fonts and then figuring out how to install them was a bit much.

Ubuntu

I've been using Fantasque sans mono for a bunch of years now.

Fira Sans / FiraGO by Mozilla, and the new SUSE font by SUSE.

Fira Code and Caskaydia Cove Nerd Font for monospace. For other uses, I'm usually good with whatever the system ships with.

For desktop, I've liked Lato, Source Sans Pro, and Inter to name three.

For terminal, I used Iosevka's customizer to create a gorgeous Fira Mono-like variant that I call Iosevka Firesque:

[buildPlans.IosevkaFiresque]
family = "Iosevka Firesque"
spacing = "term"
serifs = "sans"
noCvSs = true
exportGlyphNames = false

  [buildPlans.IosevkaFiresque.variants]
  inherits = "ss05"

    [buildPlans.IosevkaFiresque.variants.design]
    capital-g = "toothless-corner-serifless-hooked"
    capital-q = "crossing-baseline"
    g = "single-storey-serifed"
    long-s = "bent-hook-tailed"
    cyrl-a = "single-storey-earless-corner-serifed"
    cyrl-ve = "standard-interrupted-serifless"
    cyrl-capital-ze = "unilateral-serifed"
    cyrl-ze = "unilateral-serifed"
    cyrl-capital-en = "top-left-bottom-right-serifed"
    cyrl-en = "top-left-bottom-right-serifed"
    cyrl-capital-er = "open-serifless"
    cyrl-er = "earless-corner-serifless"
    cyrl-capital-u = "cursive-flat-hook-serifless"
    cyrl-u = "curly-motion-serifed"
    cyrl-capital-e = "unilateral-bottom-serifed"
    cyrl-e = "unilateral-bottom-serifed"
    brace = "straight"
    ampersand = "upper-open"
    at = "threefold"
    cent = "open"

0xProto

I find comic sans mono actually looks surprisingly nice for coding and terminal.

Lato, League Spartan, League Gothic are my three most used fonts by a wide margin. Lato and its variety of weights for most things, League when I am doing design work and need a cleaner title or header.

Lately ive been weirdly taken with TT2020 Style G, which is an odd name for a no-name font that replicates an old imperfect typewriter. For whatever reason, switching my writing software to that (Manuscript) suddenly fired up my writing flow.

I don’t have a reason to move away from the Fedora defaults except for monospaced fonts.

Terminal wise, terminus is my default. It’s so clean, and it looks good without anti-aliasing.

Roboto Mono is my current preference for monospaced fonts.

Adobe Source Code Pro and JetBrains Mono are good alternatives as well.

Ubuntu Mono for terminal, code, and data, Open Sans for the rest

Iosevka

Fantasque

Hack nerd font is my go to for terminal use.

I use M+ Fonts for most of my stuff.

I know that this will anger some people, but I just use the defaults and I don't get why there are so many fonts, since they don't seem that much different to me.

For sans-serif, I use Mona Sans. For monospace, I use Monaspace. I think it's a good-looking combination.

Anyone using Nimbus Sans?

It's actually preinstalled in a lot of systems. You can check via
gnome-font-viewer or find /usr/share/fonts -name "*Nimbus*"

I switched to Commit Mono for Terminal not too long ago but I really like it. Otherwise I use Cantarell but only because it is default and I never felt the need to change it.