I'd argue that is largely based on the type of conferences you go to. At Google I/O or WWDC, I'd agree with you in a heartbeat. If you go to a .NET conference, I'd guess you wouldn't see too many macbooks actually running OS X.
Windows developers are out there, they're just probably not attending the same talks as you are and probably not reading many of the same blogs.
Yes, but .NET is a Microsoft platform. Are there any non-Microsoft-specific platforms for which Windows is the best development environment? Because the majority of languages, frameworks, etc in the world are not made by Microsoft.
That's impossible to answer, because the definition of best is completely subjective.
Best is what works for you in whatever role you happen to be in. I know people using windows coding in RoR, Python, Java, etc with a high degree of efficiency. I know others who are doing the same thing on Linux and OS X. It works for them.
Windows developers are out there, they're just probably not attending the same talks as you are and probably not reading many of the same blogs.