Carragher like 3 years ago now probably was the best that season, had a great year, but since then has been no where near the same, last year, fair refereeing, he would have half a dozen red cards and penalties conceded, he turned into a clumsy, stupid, reckless defender. Pulling people down, throwing to the floor in the box far beyond the usual antics in the box and not as good in any other part of his game. No idea what happened to him.
Rio, is highly rated but is prone to mistakes, england and Man U towards end of the season he made so many dodgey clearances, almost scored own goals for england, and Man U several times but usually fairly solid. Woodgate seems pretty decent, but to be a "great" defender you need long term consistancy which you can't really have playing 4 games a year.
But the thing is with defenders, is singling out one player when he's part of a central defensive pair is impossible. one defender might be world class playing with one player, and seem completely average playing with someone else. Gallas/Terry would be a key example of that, 4 years ago maybe they were without question the best pair in the prem, maybe the world. Their styles complimented each other perfectly and they worked brilliantly, however neither has looked quite as good playing alongside other centre backs.
Likewise, Rio looks good, largely because he works very very well with Vidic, but its Vidic who seems to have bought that stability and consistancy to that back pair, so for me its Vidic that makes Rio seems a lot better than the other way around.
Guess my point is, its almost impossible to say whose "best" as you can only really rate the pair. Rio likes to step up, and every header he misses(quite a lot) Vidic mops up the mess, but it works very well.
Then no ones mentioned Richards, again he's not had the long termness, but the Manchester derby, Richards completely and utterly owned utd all game long, he stepped up, made last ditch tackles, was completely calm, didn't put a foot wrong the entire game and was, just brilliant. I think in years to come he might be one of the best in the world.
ANyway, clearly the best is still Adams, don't think england defence has ever looked as good since he left so he's still the best
EDIT:_ forgot about lescott, he's actually very good without the goals, incredibly strong in the air, rarely loses out in the air at corners, freekicks crosses, incredibly good. Very very good at reading the game and intercepting, stepping up and absolutely brilliant at calmly taking the ball then creating a fast counterattack.