Depends on your budget and water quality in your home. Get TDS level checked for your home water. If it's in normal range go for 2 step purification system, I would recommend to use Thailand made filters. 3rd step in filtration is B.S.
If water TDS level is high, go for RO based filteraion system, go with either Thailand or Taiwan based makers.
Water TDS level can be checked from any good filter shop, they have meter to check it.
Check water filter shop in Firdous market chowk.
Here is the number of two persons to check for rates
Shafiq is independent supplier and his number is 0321-5070122.
This is one shop on college road opposite Euro store, his number is 0321-4299178, he is also good.