EE by far. There's the odd place here and there in the Lakes where Vodafone gets voice coverage and EE doesn't, which is handy, but not enough to try and maintain a second SIM just for camping etc. And I'd say the number of times in the Lakes where Vodafone has voice and EE has nothing is far outweighed by the number of times EE has 3G or even 4G and Vodafone has GPRS or EDGE.
I'd say it goes like this:
If I wanted voice only: Vodafone, O2, EE, Three (can't beat Voda's robust 900MHz 2G network. Three comes last because it requires a 3G phone)
If I was only bothered about the best data coverage: EE, Three, Vodafone, O2
Customer service and general competence comes into it and goes: EE, O2, Three, Vodafone (atrocious)
Overall preference: EE, Three, O2, Vodafone