Gtk2 is the most mature one, Qt4 follows next (and AFAIK you can build it for Qt5 too but never tried it myself) and Gtk3 is in very early stages. Gtk1 might st

Gtk2 is the most mature one, Qt4 follows next (and AFAIK you can build it for Qt5 too but never tried it myself) and Gtk3 is in very early stages. Gtk1 might still work too, but the only distro i know that ships Gtk1 is Slackware. AFAIK Gtk2 is the preferred one for shipping since it is still widely available and you can simply give a binary (Lazarus uses a rather old version of the Gtk2 API by design so that it is compatible with almost every Gtk2 distro out there, including some that are a few years old). Qt4 and Qt5 require an intermediate shared library that provides a C API and Pascal bindings for it and in addition Qt4 is removed from some distros.