2000 was the 10th year of the 90's. If you want to argue semantics, Dojji's argument will win, because it's fact. EDIT: Though the decade is 2001-2010, not 2000-2010 or even 2001-2011. Wtf? 10 years. 2001 through 2010 is 10 years.
That said, arguing semantics for this cause is just not worth it, so whatever.
And jacko, team of the 2000s has many different definitions. If one team will be in peoples' minds 30 years down the road when they revisit this decade, the Red Sox will be at the forefront. Most successful? Yes, Yankees, but what the Red Sox did was more memorable and had more historic value, so, yes, you could argue that the Red Sox were the team of the decade. The original question was not "who was the most successful" either.