3. Boston Red Sox vs. New York Yankees

Background
Since 1919 the Red Sox have won 1 World Series, the Yankees 26. However, in 2004 the Red Sox did the unthinkable, coming back from an unprecedented 3-0 deficit to beat the Yankees in 7 games in the ALCS - after losing Game 7 of the ALCS to the Yankees the year before. The Red Sox then went on to win the World Series in 4 games over the St. Louis Cardinals.

This match-up has always had a David vs. Goliath feel to it, with the Red Sox always playing the underdog, the lovable loser. But with the World Series win in 2004, the playing field has levelled somewhat and the match-up has definitely intensified.

Why This is a Great Rivalry
History has a lot to do with this one. The Red Sox and Yankees both have long histories and their histories are intertwined with each other. In 1919 the Red Sox sold Babe Ruth to the Yankees, causing Ruth to supposedly curse the Red Sox to never win another World Series. And for 86 years it looked like it worked. This was the Curse of the Bambino - if you've ever seen the criminally-bad Fever Pitch, you should know what this is all about.

In the modern day, I'd say this rivalry hit its stride in 2003 and 2004. The Yankees won Game 7 of the 2003 ALCS in dramatic fashion, with a game-winning home run in extra innings. The story in 2004 was even more dramatic, as the Red Sox were down to their last out and were on the verge of being swept, but somehow rallied and came from 3 games down to win in 7 games.

This clip shows how intense the rivalry is for the fans, it's the scene in Fenway Park when David Ortiz beat the hated Yankees with a single in Game 5:

http://www.youtube.com/watch?v=A9ZUK4Ci3cM&mode=related&search=