Well... I would say to prefer following fuel stations (but these are based on my personal experiences, others can have different):
Caltex / Total in F-6,
Attock in H-8, and one just few meters from the entrance of Giga Mall on GT Road, when you are going towards Giga Mall from Rawalpindi side,
Shell in F-7, I-8 Markaz, and opposite DHA Phase 1 entrance roundabout, just after Al-Shifa eye trust hospital. To me, its the best in all these 3, in fact all of the above!
Others can be there too, but atm, I can only remember above mentioned fuel stations in the twin cities.