I'd say o2: they have by far the best reception in the country, have an english call centre and, if you manage to become one of their "select" customers, you get treated incredibly well: phone answered after 3 rings, friendly customer service people really going out of their way for you. In fact, I had to leave since 3 were giving a killer offer on international calls that I had to take up, but they gave me a free phone, and free 18 month contract so i keep my select customer number incase they can ever match the deal.