Monospace Font

@MacOS

@Windows

The monospace font that exists in both MacOS and Windows is "Courier New" only

The best monospace font in MacOS is "Menlo"

The best monospace font in Windows is "Consolas"

( "Lucida Console" may be better than "Consolas" in text but in user interface, "Consolas" looks better than "Lucida Console" )

The best monospace font-family is "Menlo","Consolas","Courier New" (for UI) or "Menlo","Lucida Console","Courier New" (for TEXT)

Thomas Phinney
, Typographer and Type Designer, Font Detective
Updated Dec 11, 2019
There are many:

Courier (and Courier New)
Monaco (only has regular style, not ideal)
Menlo (the previous system default, quite nice)
Andale Mono (only has regular style, not ideal)
SF Mono (newest Apple default monospaced font)
If you have Microsoft products installed, you also probably have Consolas—nominally designed for Microsoft’s ClearType screen rendering, but very nice on OS X as well.

Additionally, the East Asian system fonts are generally all monospaced. However, like Monaco and Andale Mono, they are not ideal for general use because they do not ship with a bold or italic style. Why is this a problem? most apps will let the OS generate a fake bold or italic for you, but this will be inferior to real designed bold and italic, and as you are concerned with quality, that would be an issue:

MinLiU
Osaka
SimHei
SimSunExtB
PCMyungjo
...
Note on terminology:

Microsoft-Supplied Monospaced TrueType Fonts
Summary
The only monospaced TrueType fonts shipped by Microsoft are Courier New, which shipped with Windows 3.1, and Lucida Sans Typewriter, which was included in the TrueType Font Pack. All other TrueType fonts included with Windows 3.1 and the TrueType Font Pack are proportional fonts.