sentences:  The rivalry is generally considered to be one of the fiercest in the NHL.The two franchises have been among the most successful teams since they joined the league, as the Flyers have the most Stanley Cup Finals appearances among the non-Original Six teams, while the Penguins are tied for the third most Stanley Cup wins among non-Original Six teams.The Hershey Bears are renowned for being the oldest existing AHL franchise, and the oldest existing hockey franchise outside of the NHL's Original Six.