I'm partial to GTK (it's just C and doesn't need a special pre-processor) but Qt does a much better job of looking decent on non-linux computers. There's probably some other options worth considering like libUI and Tk.
As long as you target desktop class apps, Qt is more than enough. For native mobile the offer is weaker (in Qt land, is QML) but possible. For web development Qt is not an option.
Electron can target web/desktop/mobile but the results are far from optimal (RAM, CPU cycles... JS!).
