I would probably say the Phillies, since they finished the regular season with the best record in baseball. At one point in the regular season, the Phillies looked as if they weren't going to make the playoffs. Things turned around and they finished on a strong note. With a rotation including Halladay, Oswalt and Hamels to along with a lineup including Utley, Howard and Werth, the Phillies could be a tough cookie for opposing teams in the playoffs.
Don't ignore the Yankees, though. When on top of their game, the Yankees are a pretty strong team. If Burnett could right his ship, the Yankees would be the most dangerous team right now. After all, it was the rotation of Sabathia, Burnett and Pettitte that helped the Yankees to win their 27th title last year. I wouldn't want to be an opposing team facing an effective Yankees rotation of Sabathia, Burnett, Pettitte and Hughes to go along with that powerful lineup headlined by Teixeira, Rodriguez, Cano and Swisher. Not to mention, that great Yankees bullpen of Robertson, Wood and Rivera is pretty deadly.