I have used Wx, Tk, and Gtk+. I favor Wx, despite it's problems (dependency considerations can be annoying when pacakging), largely because it allows me to write a single app that looks native on Win32, Linux, and OS-X.

Tk apps always look like Tk (which is sometimes an advantage...). Gtk+ apps rely on appropriate Gtk themes being available for OS-appropriate look-and-feel.

So, my favorite is definitely Wx. However, my second choice is Tk for the utter consistency it can provide, which is advantageous when writing the kind of apps that will be used by the same user on many platforms.