Rubbish prices, best coverage
I'd go with Three as their pricing for data is miles better than everyone else (though getting more expensive for unlimited now), but coverage is key for me especially in the more rural spots and in buildings and EE still rule there for where I use it. Plus 4G is rather nippy.
Customer service is rubbish of course but they all are.
Other attraction of Three however is they believe in Gaining Provider Led switching, i.e. where you sign up with a new provider and they do all the switching work for you without you spending hours or days battling with the existing provider's retention department just to get a PAC and then all the faff involved in the meantime switching, assuming you haven't been conned into taking a new phone contract just to keep you on their expensive tariff and as a customer.
Three were even promoting contacting your MP about it. I did and I got an obviously prepared copy'n'paste answer effectively saying the Tories have an interest in keeping it Lost Provider Led in the UK. I would assume because they have interests or investments in some of the major operators in the UK. All wrapped up in some fluff about how the UK is a unique market, blah blah, and they don't believe it's good for the UK to change it.
Bollox. And Ofcom agree with me... http://hexus.net/mobile/news/service-providers/85013-mobile-network-switching-easier-proposes-ofcom/