New York Yankees
The key beneficiary of the dismantling of the dominant Red Sox teams beginning in 1918 and ending in 1923, which has sadly beaten the Red Sox in many close playoff races, and more recently in a heartbraking 2003 ALCS, and for those reasons have earned the ire of red sox fans.  Their overspending ways, constant dominance of baseball, turning the A's and Red Sox at various points in history into virtual farm teams, and despicable owner have earned them the ire of much of the rest of the baseball world.
Go to fenway at a Sox-Yankees game, and you are sure to hear "Yankees Suck!" being chanted.(Go Red Sox)