In the UK, I would say either O2 or Orange. I've been on Orange ever since I got my first mobile phone at 14yrs, and I would say that it was the best tariff back then by quite a long way. Now, though, O2 has really good deals and great customer service, not to mention things like the O2 blue room and priority tickets for events at the arena.
For the UK then, I would probably say that O2 is best.