If I develop anything with a GUI I use GTK4. It has a bit of a learning curve to it but honestly I’ve come to like it.
I am currently creating a program for simulating networks and the drawing area is great for drawing the actual simulation because it basically allows you to have a cairo area as a widget so your possibilities there are basically unlimited and cairo is just a great drawing API.
Also gtk is basically the only modern GUI toolkit that can be used with C, which is great because it is pretty much the only language I know well enough to program a big application with. (But GObject still feels like black magic to me)