To my mind Three offer the best balance of value and coverage. I have Vodafone for work, and I have good 3G and 4G in the cities but go out a ways and you'll be suffering with EDGE or GPRS, which often means you get no usable data. I can't say how many times my phone has full signal with a GPRS connection, and I can't do email/iMessage/maps or anything, while my iPad will have one bar of Three 3G signal and it works fine. For all our personal lines we use Three PAYG, and that takes all the stupid billing and bad customer services out of the equation. Three also have 4G service for the same price as their 3G plans, which is nice. I'm a happy Three customer and would recommend them.

One thing about EE is that they have WiFi calling, which can help with poor signal, and they have visual voicemail, which if you use your phone a lot is really nice...