o2. I need a network that has reliable network coverage for calls and texts, and I have been on o2 for around 8 years now (started with them when it was BT Cellnet, then mmo2, then o2 and now Telefonica o2).
Got a great deal from them, haggled a bit to get it, will haggle some more when I upgrade in October to get an even better deal then I already have.
The network is rock solid, non of this crap waving my phone around to get a signal in a lot of places like people locally do on T-Mobile.
Tried 3 for a short period of time, the network was okay at the time, but the customer service was severely lacking, also the network was harshly locked down.
Me mum had used Vodafone for a long time before switching to o2 last year, but she never had any problems with them.
A lot of my family down south use Orange, it seems to be the network to have down there, and again no problems.
To be honest, pick a network on the basis of network reliability and coverage, forget about the deals and phones, you can get the same deals on ever network if you haggle with them.