Oh gosh, now that's a can of worms. It depends on what you want the interface to look like and where you want to take it. As far as I'm concerned, all of these interfaces are just fine.

Tk looks the dumbest (IMO), but is also the most portable (I think).

Wx is explicitly designed to be useful on multiple platforms---sometimes having multiple destination platforms in a single environment. However, not being linked to a specific GUI toolkit, it will look more or less crusty depending on the platform.

Gtk will probably look the best of all. It is linked to a single front-end (Gtk+), but your portability is limited. There is Gtk+ for Windows and Mac, but I've had mixed success with these.