Could you give some indication of what platforms you are writing for? This should weigh heavily on what is 'best'. You might find a super 'xml-foo' library that does not ship commonly on most systems by default .. while its great, the lack of the library might prevent (or at least) annoy users.
Mostly, I use libxml2 .. because its standard or easy to install on the platforms that I target.
As you see, 'best' is also determined by the library being available on your target platforms.
