I really think Neutron skin should be the default. I think most people would agree with me. It's the most convenient to use.

Disagree, by default there shouldn't be any skin at all, just pure bare-bone WinAPI, like Windows Classic

I second the Neutron suggestion, it comes in different sizes and feels reasonable enough to be the default.

WinAPI also sounds good, and would make XMPlay look similar to foobar2000. But it would probably look good only on Win7 or older. On newer Windows ... not so much, due to the absence of the Classic theme.