Unfortunately, the best GUI in existence atm is a proprietary middleware and the language for it is XAML.
NoesisGUI: https://www.noesisengine.com/xamltoy/0e2a866b60bc2b9a724b4c6...
And it even has a studio for paying clients which makes designing a GUI trivial. https://www.noesisengine.com/studio/
The studio is made using their own GUI library and its sleek af. Not even QT holds a candle to it. Would have been nice to have such a project in the open source world.