Boston Red Sox vs. Philadelphia Phillies
This hasn't exactly been new ground for either team, as each has been saddled with the other since interleague play began. Boston's more natural National League rival would appear to be the mets, especially after Bill Buckner's error and that World Series loss in 1986, but the Yankees just keep ruining everybody's good time in The Hub.
The Phillies, meanwhile, had a pretty sweet thing going before 2005, when the Washington Nationals were still known as the Montreal Expos and a rivalry series against the Baltimore Orioles was a brief drive away. Now the Nationals have stolen Philly's thunder and left it with a fairly meaningless Interstate 95 matchup with Boston each year.
Sure, the Flyers and Bruins hate each other enough to make a decent Winter Classic matchup for the NHL. Yes, the Patriots and Eagles played each other in the Super Bowl once, but Donovan McNabb's vomit pretty much nullified any animosity there. The Celtics and 76ers. Who do you think Celtics fans were asking to "Beat L.A." when the 76ers topped them in the Eastern Conference Championships in 1982?
The closest Boston and Philly come to hate for each other is when former Red Sox closer Jonathan Papelbon comes back to town with the Phillies and opens his mouth. Even then, few listen or care.