Flash

I highly recommend C++. It is the most widely used in professional game development, and thus will definitely fit all your needs. With C++, you never have to worry about the language not being sophisticated or fast enough for something, and you will never have to worry about finding 3rd party libraries to use it with. Additionally, once you are very good with C++ you will be ready to quickly pick up most any other type of language, as there are very few concepts that C++ doesn't support.

Well, although I'm a c++ almost monolingual person, I wouldn't recommend c++ for hobby game development. It took me years to figure out the language and how to use it with DirectX for example.
It's difficult enough to understand the maths, the logic and all other aspects of an API like DirectX, and using it through a COM-Interface just makes it more complicated for someone trying to learn all that stuff.

C# combined with XNA or that DirectX wrapper for C# seems fine to me (though I've never used it, just taken some looks).
About the performance part, I do not want to start a fight over this (again), but I highly doubt that using c++ over c# in a hobby project that isn't the next Farcry won't be much of a difference in terms of speed. Do it right in C# and it'll run fast :)

Another point:
Because there are dozens of libraries out there to use with c++, it's really difficult to find a specific library to do a task X.
There are dozens of xml, image, and sound libraries out there and finding the right one was difficult as well (and took alot of time).