Define 'best'. There really are only two main contenders: qt and gtk. qt sits on top of C++ and gtk sits on top of C, so your preference may be dictated by that unless you work equally well with both C and C++. of course, there are also C++ bindings for gtk (gtkmm).
qt benefits from a well-developed IDE - qtDesigner (IIRC). But qt is heavier than gtk, although gtk is pretty heavy these days.
If you have a mind to writing software which should be portable to windows, then qt/C++ is definitely the way to go.
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