I suppose the best network choice reflects your location, the network used by friends/family/colleagues, costs pertaining to SMS and data.
Living in Islamabad, Ufone is probably the cheapest for calls as most of the population is on that network. However, they have given me multiple issues like inability to hear the caller's voice intermittently, lack of decent 3G coverage, relatively high 3G pricing (remember how they priced on the basis of speeds? Unbelievable! They're still relatively expensive), inability to open google.com even with a 15 Mbps speed test (no kidding!). I haven't noted any visible deductions in my Ufone balance without my knowledge as have you, however.
I then shifted to ZONG as they seemed to have the fastest speeds during their initial 3G rollout. Unfortunately, my biggest gripe was apparently no 2G signal in my own house! It has been 3 years and cousins with ZONG sims that come to visit me have to get next to the windows otherwise, they'll lose connection. Other then that, I had poor voice quality and my balance tended to keep decreasing without any usage at all which was rather infuriating considering their other problems.
And now I'm on Telenor using their my djuice offer package which probably has the best pricing of all; 50 off-net minutes, 1500 MB, 10,000 SMS for 142 rupees.
Telenor probably has the worst internet quality of all networks with constant timeouts and failed downloads; I guess you get what you pay for.
I can't say much for Mobilink/Warid as I haven't used them. But I would believe it is quite expensive comparatively due to few users in the twin cities. But who knows, I might just get one SIM and try them out..