Three candidates:

MaCaw is firming as a great and has owned the last part of the decade.
Umaga had universal approval as captain and was unquestioned.
Thorne was mostly maligned and always questioned.

It would seem a two horse race between MaCaw and Umaga both with un questioned leadership and results:

Umaga- Lost 2004 tri nations, Won 2005 tri nations, Retained Bledisloe cup, Whitewashed Lions, Grand Slam. Winning % as Capt 86%(18 from 21)
Not capt in a world cup Winning % as Player 80%
MaCaw-Won 2006-2008 tri nations, Lost 2009 tri nations, Retained Bledisloe cup, unbeaten in the north. Winning % as Capt 86%(37 from 43)
Grand Slam, Lost in Quarters of World cup Winning % as player 87%
Both very impressive, but how does Thorne compare?
Thorne- Won 2002 & 2003 tri nations, Regained the Bledisloe cup, unbeaten in the north, Winning % as Capt 87%(20 from 23)
Lost in semis of world cup. Winning % as player 85%

All of these players have outstanding winning % records as capt, comparable with the best of all time (Fitzy,Winneray etc), Thorne's was the best at the end of the decade. None won a world cup but there aren't many All Black Captains that have! McCaw and Thorne captained at world cups this decade, Thorne achieved the better placing.
Umaga achieved a whitewash over the Lions a great achievement but in reality a series against the French would have been a much better contest, ditto the Grand Slams. Umaga won one Tri nations and lost one. Thorne won two and achieved the biggest tri nation winning scores against the Wallabies and Boks - both 50 points plus, Thorne never lost to South Africa as captain 5-0.

So in my opinion it would comedown to MaCaw and Thorne, if Umaga had stayed around for another couple of years it would have been different, but he didn't..
There is no doubt that MaCaw will be one of the all time great captains and the thing that sways his way at the moment is the amount of games he has played as captain.
But biased as I may be I'm going to give it to the man with the best winning % of the decade, the best word cup placing and the captain who subdued the mighty Bokke in the noughties.

Congratulations Mr Thorne!

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