As I said earlier, it's very different for everyone, so you can't really say what is the best or not, in my area, 3 are pretty awful, as are Vodafone and Orange, but O2 and T-Mobile are far better.
I know T-Mobile and 3 are meant to share the same masts, but obviously not all of them
I know T-Mobile and 3 are meant to share the same masts, but obviously not all of them
