Given his performence behind a midfield with no concern or whatsoever over defending, Charles Puyol must be the top guy. However it is not easy to call the best among the likes of Ferdinand, Vidic, Terry, etc...
Terry, Vidic, and Ferdinand are overrated. I am talking of who is the best right now. Zanetti has an elegant, solid, and efficient way of defending. Ofcourse, he doesn't always play defense. But, that is another quality that makes him great. I don't think Ferdiand, Vidic or Terry can play center-mid, or outside mid.
I guess the problem with your question is defining what you mean by defender. A full back has different duties to the centre back (very often more thought of as an attacking player than a defender). :-) Ramos would get my vote as the finest full back of the moment. As far as central defenders go, it also depends on who they're played with. John Terry, for instance, is decidedly average when played next to Alex because they are too similar. But when Terry and Carvalho play, they compliment each other far better. To my mind, it's always better to play one solid stopper and one ball playing sweeper type: Terry/Carvalho, Vidic/Ferdinand, Charlton/Moore. :-D And that's only talking about a back 4!
What I was getting at was that you would have to qualify your criteria for what you mean by a defender. As a for instance(and I'll stick to Manchester United here), Vidic and Rafael are completely different types of players, with Vidic being very good defensively and Rafael being better at going forward. They're not really comparable, yet they would both be termed defenders If you're talking about who contributes more defensively to your team, I would argue a midfielder in the Makelele or Galasek role would be more useful than a Rafael.
If we're strictly talking about players whose key contribution is defensive, it would have to be one of Vidic, Ferdinand, Puyol, Kaladze, possibly Bordon, for me. I'd be leaning towards Vidic at the moment. :-) Carvalho would be on the list, except for the fact he's not played for a while.
