Having worked with a variety of GUI frameworks on small projects for work (WPF, Windows Forms, Xamrin, Android, iOS), I think HTML and CSS are the best out there for easy of use. It definitely helps that it has so much history and documentation behind it, but if I could build my GUIs in standard HTML and CSS without being forced into Electron or something else and still have my choice for a backend language, I'd find building GUIs way more fun.