The alternative for Visual Studio IDE to run C# programs −
SharpDevelop
It is an open source IDE to run C# programs but works only Microsoft Windows. SharpDevelop was developed as an alternative to Visual Studio. It is written in C#. Supports Git, SVN, NuGet,
LINQPad
It is a utility that allows you to run C# programs without having an IDE. Some of the features in LINQ Pa d is paid like autocomplete and allows the user to access it only after the payment.
Monodevelop
It is an open source IDE that allows you to run C# on multiple platforms i.e. Windows, Linux, and MacOS. MonoDevelop is also known as Xamarin Studio.
Monodevelop has a C# compiler to run C# programs.
I would consider the best IDE for C# besides Visual Studio is SharpDevelop.