Both franchises had been the dregs of the NHL in the early and mid 1960s. They got better in a hurry after the first wave of expansion. When they got jammed together in the East division in the early '70s it got pretty intense. Add in the 1970 playoffs, the 1972 Stanley Cup and the 1973 playoffs (at least the Rangers won one of those matchups). There were five great teams in that 1967-75 expansion era - the Canadiens, Bruins, Rangers, Blackhawks and Flyers. The Rangers and Blackhawks conspired somehow to win zero Stanley Cups between them despite each having a core of Hall-of-Famers.
As a Rangers fan I figure they can't beat the Islanders enough to satisfy my revengelust. Then there's the Flyers and Devils. The Bruins fall into my third tier of hockey enemies with the Penguins and Caps.