I have found 3 generally quite good.
They have been the best network i've used so far to be honest, at first a few years ago, the network coverage so bad that I quit just after a week when they launched pay as you go, I came back to them last year and the network coverage has improved vastly.
Here's my experience of the other networks and why I wouldn't go with them again:
O2: Poor 3G coverage and expensive tariffs
Vodafone: Bad speed for surfing the net on your phone,expensive tariffs as well.
T-Mobile: Poor 2G and 3G reception, dropped calls all the time,bad customer relations,bad complaints procedure and big bunch of liers! Selling me a service when they knew I was in there 3G blackspot.
Orange: Once had a sim only deal with them, they gave me someone elses number which was in use 3 months before they gave it to me according to Orange and also had problems with incomming calls from people calling me from the T-Mobile, when people tried to call me they just heard that they dialled an incorrect number, and there network coverage is not as good as they claim. Unreliable!
Virgin: Actually a very good service especially when it came to customer service but unfortunetly they use T-Mobile's network and only got 3 bars of reception.
Now with 3 I haven't experienced any of that apart from the odd time the reception went low and then used the 2G reception for a short while, but when it comes to reliability and accurate billing and value for money on calls, 3 works out best for me and they offer services such as Skype and Windows Live Messenger and free calls to other 3 mobiles which I find very useful.
Ok they may use an indian call centre,but most companies do, they speak English,they deal with my enquiry they are human beings, thats it, what more do you want from a call centre,what difference is a call centre over here, i've experienced poor customer service a lot from UK call centres.