I used to like MonoDevelop maybe 10 years ago, but it's not around any more. If I remember correctly, it was the only open-source IDE that supported C# and ran on Linux. That was before C# and .NET were open-source and Mono was the only way to run C# apps on Linux. Things are way different now.
The best today is obviously nano
. It has syntax highlighting, auto-indentation, and at some point they made it so Ctrl+S saves the file. What more do you need? (cut and paste still use weird shortcuts though)