C# Blog - Tips and Tricks: Add Syntax Highlighting Control:

After searching i found editor control from a well known open source IDE SharpDevelop. Syntax Highlighting Text Editor Control looks very good and light after Scintilla. It requires less time to load and seems much more stable. 

First, what i have try to use, was ScintillaNet, but i have no luck to get it worked in test program. It need to setup before use, require unmanaged companion library, what make difficult to multiplatform use. After all i see my Visual Studio C# 2010 Express very unstable, after adding ScintillaNet control to Toolbox. After many experiments i get it worked, but result seems unstable and unrepeatable. Personally i not recommend to use Scintilla in .NET application, but possible it good choice for C++ project.

There's a StackOverflow thread on this question, and the ICSharpCode lib, especially with the excellent sample project at the above link, seems the most portable (though its answer had no votes until I came along).

Works well with simplest case tests.