Well the best so far for me is TM CNG on ISB Expressway but listed below are very good too:
1) Shell Pump Defence Chowk( Average: Used to be pretty good initially then quality fell but discount creeped in so was pretty usefull even then but now with no discount NOT RECOMMENDED)
2) Caltex CNG After swan bridge adjoining Pak Suzuki Motors ( GOOD: Recommended but do not buy any thing from their shop its Over Priced :P)
3) National CNG after Swan Bridge on left( Average)
4) Al-Basit CNG after leaving Bahria Town Phase 8 entrance on g.t road( Ver GOOD these days my destination for CNG nowadays)
5) PSO PUMP Jhanda Chichi: (GOOD)
ALL in ALL i havent seen a cng pump mainting its quality over a long period of time. PSO Jhanda has to be acclaimed for maininting its quality uptill now.
If resident in the whereabouts of swan bridge i can fully recommend al basit cng now a days but do keep a watch and if u feel depretiation in quality u can switch to any other mentioned above.