Before asking what is the best tool you should probably define what problem you're trying to solve.

• Should it really be cross platform?

• Why C++? For performance? To integrate with existing libraries? Because you only know C++?

• Do you want it to be flashy and cool (e.g. games) or standardized and information dense (e.g.: corporate applications)?

• What do you want from said GUI? Customization, small code base , standardized and consistent native controls, easy to change or adapt?

From C++/WinRT and C#/.Net to QT and wxWidgets there are a lot of options, each one with its specific trade-off and advantages. It helps to know what your priorities are in order to give you an answer.