Some rivalries in the NHL are decades-long; think Boston Bruins-Montreal Canadiens, Chicago Blackhawks-Detroit Red Wings, New York Rangers-New York Islanders, etc. But some of the most fearsome in the league have materialized more recently, such as the one between the Florida Panthers and Tampa Bay Lightning.
The two Florida teams would seem like natural rivals, but the clubs didn't hit peaks in their competitive cycles simultaneously until the past few seasons. And now, their games are simply must-watch for all hockey fans, period.