I have to disagree with Ramos being one of the best. He is definately one of the best RB's in the world but he gets caught out of position far too much at CB, though he is still young so he could definately get better.
I think Rio is definately the best CB in the world atm. Unfortunately Terry is/will be the one who gets more credit from the press because the english media prefer a player who appears to have more heart than talent.
3 or 4 years ago he used to be fairly mistake-prone or have lapses in concentration but in the last few years he has matured and is now not only playing at an incredibly high level, but he has turned into somewhat of a leader. Like Pally said Vidic definately brings out the best in him because Rio's only flaw is that he doesn't 'stick his head in' in the way that Vidic does, though he has improved a lot in this respect.
For me his performances against Liverpool away, Roma away, and the two Barca legs were as good as any performance I've seen from a Manutd defender.
He has definately 'stepped up' since Gazza has been injured, hopefully he'll maintain his leadership because with all due respect to Gazza, I think Rio is looked upon as the leader now. You saw at the players awards Ronaldo, Anderson and Nani point to Rio's picture and praise him.
The top 2 CB's in the world IMO are: Rio and Nesta (when fit).
After that theres a list of players who are very close and all have an arguement for being the 'best of the rest.' Calvalho, Vidic, Cannavaro, Lucio, Mattarazzi (before E08), Gallas, Terry, Bramble..just kidding
Now that I actually sat and though about it there aren't that many top quality CB's in world atm, although the likes of Richards, Garay, Albiol, Agger, Ramos could/should turn into some of the best.