Personally I find that KDE, gnome and xfce have great UI, you can find their guidelines in the list above.
Regarding implementation I'm a big fan of Qt, there's an excellent python binding as well (PyQt), which makes it easy to get started especially if you already have your logic implementing in Python.
If you want something that isn't tied to Apple you can find a list of similar guidelines here: https://en.wikipedia.org/wiki/Human_interface_guidelines#Exa...
Personally I find that KDE, gnome and xfce have great UI, you can find their guidelines in the list above.
Regarding implementation I'm a big fan of Qt, there's an excellent python binding as well (PyQt), which makes it easy to get started especially if you already have your logic implementing in Python.