Note: You ask different people, and they will all have their favorite tool kits. GPUI is very promising but new. There are a pile of others people will vouch for like Druid, Tauri (Electron-like but faster?) Slint (As the sibling poster pointed out), Iced, Qt bindings etc. I am not sure about native Windows etc bindings: I recognize that OS-native things like that are the most performant, but for me it's worth it to make my programs cross-platform and not worry about OS-specific things.
Note: EGUI is an immediate mode GUI, which isn't for all applications. But it has never limited me, and have written some complex UIs with integrated 3D etc.
Note: You ask different people, and they will all have their favorite tool kits. GPUI is very promising but new. There are a pile of others people will vouch for like Druid, Tauri (Electron-like but faster?) Slint (As the sibling poster pointed out), Iced, Qt bindings etc. I am not sure about native Windows etc bindings: I recognize that OS-native things like that are the most performant, but for me it's worth it to make my programs cross-platform and not worry about OS-specific things.
Note: EGUI is an immediate mode GUI, which isn't for all applications. But it has never limited me, and have written some complex UIs with integrated 3D etc.