Swindon station is the home of most of my trainspotting. For a start it has good visibility, great waiting rooms as they are glass and you can see almost all of the trains from a warm and cosy waiting room. Facilitates are a given for a mainline station.
Overall I would highly recommend this station for trainspotting (because of all of the interesting freight that passes the station.)
DON’T forget to look at platform 1 which is tucked away at one end of the station.