I would go with Visual Studio 2010 Express for Windows Phone. You can download it here:
http://www.microsoft.com/visualstudio/eng/downloads[
^]
Also, I would recommend wp7.1 because it will also be compatible with later versions up to wp8. Forget about 6, I literally threw my phone away because I couldn't bare selling it to anyone else and pass on the misery it gave... wp7 is, although a little late in the market, the first real usable windows phone.
Good luck!