Personally I am kind of addicted to Fira (both for terminal and UI), but I have also used Robot a lot. Right now I am actually using Liberation for both and it looks real good too.
Whatever is default.
Pudge
In terminal it is Source Code Pro semibold 11pt.
Everybody loves that one and I find it both ugly and hard to read.
Pudge is more sensitive about what the font is writting than the font itself. Would be the same to play guitar and speak more about his color than the sound it goes from !
I have to admit as a guitarist at my younger time, I was according importancy for the color, where now sound is the only necessity. 
I use Mono Sans 13, I think is the default one !
Terminal/Konsole - Noto Mono 13pt
System (et al) - Noto Sans 12pt
Fixed - Hack 12
I think the same !
Otherwise for windows and the rest of the system I use Ubuntu condensed
Yep, I checked it now, it’s exactly Monospace regular 12 the default XFCE.
Fira Code was quite recently released at version 2.0, with a LOT of improvements. I cannot recommend it enough.
I’m not a real font connoisseur. I just click on fonts until I see something that I like. If my eye likes it, I like it. The only differentiation I make is between fixed and proportional.
Noto Sans for GUI, and I generally leave the terminal font set to the default,
One thing that I regret to admit is that Apple knows what they’re doing.
I tried out the default OSX fonts: San Fransisco and New York (the new one)… And I have to admit that when it comes to readability, nothing beats SF Pro Text (the name of San Fransisco Sans in the font list).
It is noticeable better than Roboto, and close to twice as clear as say Noto or Liberation.
The only question is: Do I want to have Apple fonts on my Linux PC…
Inter (system-wide)*
Ubuntu Mono (Terminal)
*how would I change the system font (from Cantarell) to Inter, including the top bar (in GNOME)? Not available in gnome-tweaks as of yet.
I would like to bring some history about FONTS. The base of all the fonts that we are using today comes from the Germany design school BAUHAUS around1930, CarlMarx, Reross, Alfarn etc…
They have the particularity of sans serif as Swiss 721, Futura, Helvetica…All the google fonts Noto, Roboto etc… results of these references fonts…