Hockey is an emotional sport as well as a physically punishing sport. These two factors, combined with the fact that teams play each other up to six times per season (not including playoffs) year after year, causes rivalries to heat up in a hurry. Of course, back in the day when just six teams played in the NHL these rivalries were fierce among all opponents. These days, in today's 30 team league, many match-ups just don't have the same spark. How can fans or players get pumped for a Minnesota Wild/Florida Panthers game. These two teams hardly see one another, and basically have no history.

Yes, history plays a huge role in rivalries. Grudges go back generations in some cases, and fans, media and players remember any slight, real or imagined against their team. That said, nothing builds a rivalry more quickly than a hard-fought playoff series, and teams that had no traditional rivalry can suddenly become bitter enemies. The Detroit Red Wings/Colorado Avalanche feud of the late 90s is a perfect example of this.

So who are today's most bitter NHL rivals? Read on as I count down the top 5.

5 - New York Islanders vs. New York Rangers

Hockey's version of the Subway Series is a feud that will never die. These two teams, and particularly their fans, love to hate each other. The peak of the rivalry came back in 1979 when Isles' defenseman Denis Potvin hit the Rangers' star forward Ulf Nilsson. Nilsson's skate caught in a rut in the ice, and the Swedish star ended up breaking his ankle. Rangers' faithful were enraged, and the chant "Potvin sucks!" and many less flattering chants have been staples in Madison Square Garden ever since. To add salt to the wound the Isles went on to win 4 straight Stanley Cups in the years that followed, while the Rangers' fortunes sputtered. New Yorkers who weren't born in 1979 have been told the tale of the Potvin hit and the rivalry hasn't diminished much, if at all, in the more than 30 years since.

4 - Edmonton Oilers vs. Calgary Flames

Though today's rivalry is but a pale shadow of the one that existed in the 1980s when both of these teams were Stanley Cup contenders, the Battle of Alberta continues to rage both on and off the ice between these provincial rivals. No, Jarome Iginla vs Taylor Hall isn't in the same class as Mark Messier vs. Joel Otto, but despite the fact that neither of these teams has been a force in recent years they still manage to get the emotion ramped up to face one another, and one head to head match-up in the playoffs is all it would take to rekindle the coals on this legendary match-up.

3 - Pittsburgh Penguins vs. Washington Capitals

Probably more than any other rivalry on this list, this particular feud is centered around two players. Ever since they broke into the league together in 2005-06 the debate has raged whether Alexander Ovechkin or Sidney Crosby was the NHL's best player. The jury is still out, but Crosby and Ovechkin have dragged their teammates into the feud, and every time these teams clash the result is fire on ice. So far the advantage has gone to the Pens as they have been victorious in the two head to head playoff match-ups the clubs have had in recent years, but Washington looks very strong this year and with Crosby's health in doubt the pendulum could finally swing in the Caps favor come the post-season.

Pittsburgh actually has a couple of bitter feuds going, and I could just as easily have put Pens/Flyers in this spot. However, the departure of Mike Richards to the Los Angeles Kings dampens the fire a little bit, so Pens/Caps got the nod.

2 - Montreal Canadiens vs. Toronto Maple Leafs

I had to have an "Original Six" match-up on this list, and it doesn't get any better than the one that exists between these two ancient Canadian hockey rivals. Habs/Leafs have been battling it out for close to a century now, and regardless of which team is stronger in any given year they always seem to match up evenly when they face each other. They've played 707 times in the regular season going back to 1917 (including St. Pat's/Arenas days), with Montreal holding the edge 333-279-88. They've met in 15 playoff series (8-7 Montreal) and 6 Stanley Cup Finals (4-2 Toronto). The two teams haven't met in the playoffs in decades, but should fortune pit them against one another once again buckle up for one of the greatest series in recent memory.

1 - Vancouver Canucks vs. Chicago Blackhawks

As legendary as the Montreal/Toronto rivalry is, they are currently "Out-Heated" by the feud that currently exists between the Canucks and Blackhawks. These two squads have been at each other's throats for the last three seasons, most particularly in the playoffs where they've met in three consecutive seasons. The Hawks were victorious in each of the first two bouts, going on to capture the Stanley Cup after dispatching Vancouver in the Western Conference semi-finals in 2010. Vancouver got their revenge last year, beating out the Hawks in the first round, but it was a near thing. After building what they thought was a comfortable 3-0 series lead the Canucks faltered and the Hawks stormed back, pushing the series all the way to OT in game 7 before an Alex Burrows goal finally exorcised the demons of the Canucks' past playoff failures.

Though the Hawks have lost some of the pieces that made the rivalry special, particularly Dustin Byfuglien and Andrew Ladd, these two teams still have a healthy hate on for one another. After the way the first round series finished up last year, look for there to still be plenty of fuel on the fire that exists between the two clubs.