South Carolina.
Quite honestly, it's just the biggest rivalry in professional sports. I was first drawn into it in the '99 playoffs, when I was getting back into baseball again and heard all the commentators talk about the rich history of the rivalry and started watching pieces on ESPN Classic over the years. I was geeked for this year's ALCS, and promptly saw one of the best, if not the best series I've ever seen, with LOADS of legitimate aggression on both sides, a whole country tuning in, a thrilling extra innings Game 7 walk-off homer finish, all that. I favor the Red Sox over the Yankees simply because I want to see the poor guys finally beat the Yankees and win a World Series. Same reason I rooted for the Cubs. Brothers gotta finally win sometimes.
'course, as soon as football season starts up, I'm all about Packers/Bears, but that rivalry is always one-sided one way or another, unlike the recent equality of Yankees/Red Sox.