In my opinion, you'd have to argue pretty hard that he's the best English centre-back. In fact, I'd say he isn't even the best English centre-back at his own club, let alone the country.
I think that between Ferdinand's injuries and his declining pace, it's probably time to bring new blood into the setup. He's also continuing with his bizarre obsession of playing about 10 yards deeper than any other defender, which really doesn't help matters. Moreover, Ferdinand's lack of pace wouldn't be such a problem if he could play a more advanced role, the stopper rather than the sweeper if you will. However, not only is it not his favoured position, but we've got better defenders suited to it and crucially only one who has the speed and reading of the game to properly sweep (Jagielka).
As such, my dislike of Ferdinand for England is based mostly upon horses for courses, not necessarily player quality. However, I'd be intrigued to see how he offers more to a squad, in your opinion, than the versatile Kelly.