MonoGame is a drop in replacement for Xna that seems to work ok. It's getting a fair bit of attention for Windows 8, and has attracted some developers to continue development.
Xna doesn't seem to be supported by Microsoft any longer, you can still use it while MonoGame gets developed further though.
Wpf is easy to use and it's built into .NET, but it doesn't allow easy access to custom shader support, so you can't get the fanciest lighting. It's easy to use for CAD though.
SharpDx and SlimDx seem popular, I haven't used them a whole lot though. I tried OpenTK briefly and it seems to work ok, if you prefer OpenGl over DirectX you might like it.
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