Lets say 2007 all star team vs 1977 all star team.
There are a few important things to consider.
1. Like Sens said, equipment is crucial. Players' equipment is lighter and more streamlined today, and goalies' equipment covers much more of the net. If modern players had the advantage of using current equipment, and older players had to use older equipment, modern players would definitely win. But that says more about the importance of equipment in hockey than it does about the quality of the players. It's certainly possible that Bobby Hull, with with a light, composite stick, could score 60 goals in today's NHL.
2. What officiating standards would be in place? There's far more interference and clutching & grabbing in the modern NHL than there was prior to the nineties. If the refs called the game the way it used to be called, the modern team would spend all game in the penalty box, and the older team would surely win.
3. How would roster sizes be adjusted? This is a problem if you go back to, say, the 1950s and earlier. In the past, roster sizes were considerably smaller, thereby forcing stars to routinely play 30-50 minutes per game. Naturally this tires them out faster and makes them more prone to injuries. I wonder how Niedermayer & Pronger would fare playing 45 minutes
every game, or how Selanne & McDonald would do playing 35 each night.
One factor that I think is largely irrelevant is size (height and weight). Martin St. Louis (5'9", 180 lbs) won a Hart and Art Ross playing against Zdeno Chara (6'9", 260 lbs) and other enormous defensemen. Surely players like Howe, Hull, Horton, Esposito, and many others, are physically large and strong enough to survive in the modern NHL.
I realize I haven't really answered your question. It's a tough question because there are so many important factors to consider.