What is the best default font to use for a Unicode Application in which the user can select the font he wants to use?
The problem I notice is that not all Windows machines have every Unicode font. And every Unicode font does not include all the Unicode characters.
So what would be the font that would have the best trade off between availability and completeness to use as the default font? Take off marks if the font is ugly.
Then, after the default font is decided on, what fonts should be included in a dropdown list for the user to select from?
The best default font for any Windows application, Unicode or not, is the one that the user has chosen in the system control applet for all applications. Nothing is more annoying than having to configure fonts and colours in each and every application, just so that they match the system defaults and don’t visually clash with other applications.
Besides: I think it can be reasonably assumed that the system default font supports all glyphs of the supported languages (note that various Windows versions do install different subsets of the available language support, and for testing it may be necessary for you to manually add support for more languages). And if the user has changed the default, then most probably they have chosen another font that is able to show all glyphs.
As for your second question, why not leave the decision to the user, completely? What reason would there be for not including all?