I've been thinking about tinkering with 3D graphics programming in .NET. In the past, I've thought about Truevision3D, and XNA, but I've not used either of these. I scanned Stackoverflow for the exact question, but neither of the (almost) relevant question (such as this question about rendering graphics, and this question about Learning Game Programming) answer my specific question.

Out of the graphics engine APIs you've used for .NET, which is the easiest to use, which has the most features, and which is the cheapest? Which would you recommend for a .NET programmer to learn first?