That depends on what you need. The regular C# IDE would be Visual Studio, and for mobile development you could use Xamarin within Visual Studio. Xamarin makes your project cross-platform, so you could release the app on both Android and iOS while only coding it once. If you want to make Android Games you're better off learning Unity and pairing it with Visual Studio. Unity also publishes your end result as a cross-platform app, so code once and play everywhere.
