Python 3.6.5 without a doubt. To install, follow these tutorials (I recommend not to do it any other way, as these tutorials are complete and correct, except perhaps python version number (currently 3.6.5)):
python 3.6 install here
for simple GUI you can use tkinter
If you are going to be using more comples GUI window (and it appears that may be possible) I would recommend wxpython (phoenix) or Qt5 (pyqt5).
For your serial communications, you can use pyserial.
once you have python installed, you can install packages using pip
python 3.6 install here
for simple GUI you can use tkinter
If you are going to be using more comples GUI window (and it appears that may be possible) I would recommend wxpython (phoenix) or Qt5 (pyqt5).
For your serial communications, you can use pyserial.
once you have python installed, you can install packages using pip