Andy Schleck of Saxo Bank. He showed amazing strength on l'Alpe d'Huez last summer by starting a number of fake pursuits that wore down Cadel Evans and Bernhard Kohl, helping Carlos Sastre maintain his solo breakaway.
The team consists of some incredibly strong support riders like time trial superstar Fabian Cancellara, who rode at the head of the peleton for about an hour in the early climb up the mountain last year and who dropped a large number of riders while doing so. He'll be a valuable asset for Andy Schleck, whose only weakness is his youth.
Another strength for Saxo Bank is their cohesiveness, and the same guys are back from last year (minus Sastre). Astana has riders just as strong as Saxo Bank, but they might not play together as well.
Saxo Bank won the Tour not just because of strength, but because of shrewd tactics. During the Alpe d'Huez stage, Carlos Sastre went back to the team car to fetch water bottles for Frank Schleck, who was in the yellow jersey. So the contenders were caught off guard when Sastre later took off up the mountain.
Guys like Cadel Evans, Alexandro Valverde, Denis Menchov, and Christian Van De Velde were lured into fake pursuits by the Schleck brothers, who would move sideways while accelerating to make the other guys waste energy trying to stay on their wheels.