I'll get shot for suggesting this most places, but I feel like GTK has the most 'native' feel across every operating system. That's not to say that it's easy to get working (setting up proper build targets is going to be your biggest hurdle), but your reward is a stylish and robust UI with proper touchscreen support on Windows/Linux.
I personally think the exact opposite. GTK needs a lot of custom styling to get the "native" style to work, and even then it's kind of lacking.
GTK feels right at home in the GNOME desktop environment (which I use, and like!) but for cross platform I don't really think it feels native at all. As a user, I've had much better native experiences with QT or WxWidgets to be honest (on Windows, at least).
