I cant comment on Ayala or Chivu because i hardly saw them playing.
Of the other three you mentioned..
Thuram - Since transfer to Juve, he, IMO failed to justify his price tag - with this season being exception. This season he is performing up to his ability and Juve are getting their value for money now. Stil, i think that this is still not at the same level like three, four years ago.
Canna - Solid. It is not close to Canna at Parma though. It is his first season with new team so it is to be expected. I have feeling that he is getting rougher & rougher as the times goes by, but maybe I'm wrong.
Hyppia - He is rock solid, agreed. His height gives him great advantage in the air, and obviously it is a great advantage on corners situations where he is as dangerous as any striker and needs covering.
Now, i find it highly unjust to justify title "best" based on one year performance only. This year, agreed, Nesta probably can not be considered in the top five. He changed the team. he's been injured over & over again. Add to that traum of his departure from Lazio = lower than expected performance.
Secondly, Nesta in Lazio had total organisation of defence in his own hands. Ie, you could see it regulary that he is positioning them around the pitch and affects their play immensly. Lazio's admittedly mediocre defenders like Gottardi, Negro and Co. always looked solid, and great part for this belongs to Nesta's vision of the game. In Milan, he has a figure of Maldini to deal with. Give Nesta free hand to "organise" Milan's defence and you'll see the improvement. (Ironically, Maldini's is going through rejuvenation this year. His performance has been simply great. Playing next to Nesta, he has a thing to prove, i guess

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Now, Note that Nesta is youngest of these three and that his performance over last 5 years is in the least equal if not better to them. At any point in time i would chose Nesta in my team over these three. And I'm not being subjective. Whilst IMO Canna is the best man-marker, there is no defender that can tackle from all the possible angles like Nesta can. There is no defender i saw playing that can control his team defence like he can, nor such a vision of the game.....at least at this point in time.
still, this is only my oppinion. Some PPL would chose Campbell over Nesta
