Depending upon your motivation for monospace, the iA Writer site offers my current favourite: IA Writer Duospace.
If you want monospace for the crisp, clear, draft-style, clutter-free look when writing code (or similar) then it's excellent. If you absolutely must have every character being exactly the same width then less so as the main difference is that it is monospace for everything except 'm', 'M', 'w', and 'W' which are each 1.5 wide (so it takes two to bring things back in alignment). In practice that is often less of an issue than it seems, and the benefit in readability/clarity/looks (literal mono for those characters makes them compressed) makes it worth giving it a try, even if you only end up using it for Markdown files etc.
If you want monospace for the crisp, clear, draft-style, clutter-free look when writing code (or similar) then it's excellent. If you absolutely must have every character being exactly the same width then less so as the main difference is that it is monospace for everything except 'm', 'M', 'w', and 'W' which are each 1.5 wide (so it takes two to bring things back in alignment). In practice that is often less of an issue than it seems, and the benefit in readability/clarity/looks (literal mono for those characters makes them compressed) makes it worth giving it a try, even if you only end up using it for Markdown files etc.
They have a web page explaining it (alongside a download): https://ia.net/topics/in-search-of-the-perfect-writing-font