Open Source or Free fonts are appreciated.
Apparently, Frutiger said that "Helvetica is the jeans, and Univers the dinner jacket." I think that's appropriate.
I'm also fond of FF Scala and FF Scala Sans: https://en.wikipedia.org/wiki/FF_Scala
https://en.wikipedia.org/wiki/FF_Scala_Sans
Sadly, these are not open source. But they all have a relatively large number of glyphs.
If you want something open-source and with a ̶H̶U̶U̶U̶G̶E̶ largish number of glyphs, here is Junicode: https://github.com/psb1558/Junicode-New
* Serif: Bitter (https://www.huertatipografica.com/en/fonts/bitter-ht) or maybe Vollkorn (http://vollkorn-typeface.com)
* Mono: Iosevka
* Sans-serif: maybe Fira Sans Condensed, previously perhaps DTL Prokyon.
I haven't seen any sans-serif fonts I've really loved in recent memory. I feel like most sans-serifs just influence from Helvetica and are really boring, lack character or creativity. I'd love to see if anyone here has any examples of novel or interesting sans serifs.
(I also know that good typography doesn't distract from the content, but even under that ideal I still find most modern sans serifs boring).
https://en.wikipedia.org/wiki/Computer_Modern
Note that the plain serif font in Computer Modern is a beautiful font for high resolution screens or printing, but has thin serifs not as suitable for low resolution at small font sizes.
Deja Vu is also open, and pretty nice for screen usage.
As for non-free fonts, I love Gill Sans.
To a lesser extent IBM Plex - https://en.wikipedia.org/wiki/IBM_Plex
And a genuine classic in the the Eurostile family https://en.wikipedia.org/wiki/Eurostile
Look at that gorgeous capital R!
http://www.identifont.com/similar?6WX
I like Lemon/Milk too
https://www.dafont.com/lemon-milk.font
And I'm an absolute fan of Futura
https://en.wikipedia.org/wiki/Futura_(typeface)
The mix of Lemon Milk and Futura is awesome
Scala and Scala Sans already mentioned upthread
Quadraat Sans: https://www.typeby.com/fonts/quadraat-sans
Source Code Pro (mono) in particular is my day-to-day workhorse, but I also use Source Sans. Not a huge fan of Serif fonts truthfully.
Iosevka family comes in a close second.
https://blog.golang.org/go-fonts
They are clear, unambiguous, and pleasing to the eye!
For sans-serif, I want fonts to be playful and cute and fun - and Fira Sans is exactly that.
For monospace fonts, I just use Iosevka and Source Code Pro, because they are pretty in my eyes and most of all get the job done.
Nothing comes close.