This comes down to three players for me:
1. Mario Lemieux
2. Peter Forsberg
3. Pavel Bure
Mario because he was my favorite player in NHL to watch play--it was like watching The Beatles vs. a bar cover band on most nights--Mario was that great. I'm of the opinion, if he entered the league today, he would put the NHL through a shredder, based on his size, skill and hockey sense. He would train and eat like today's players and his physical gifts were just better than anyone...ever.
Forsberg would be an absolute monster today. It wasn't "that long ago" that he was at the end of his line, playing with busted feet, and still giving a young Sidney Crosby a run for his money when they matched up head-to-head. Forsberg's wizardry would be on full display in today's game and he would bring a physical, nasty element that other megastars couldn't. The best way I could describe Forsberg in today's game would be if you took Patrick Kane's hands, added it to Matthew Tkachuk's physicality and viciousness... then put it all in David Pastrnak's body.
Simply put: Bure was tailor-made for today's NHL. He could fly as fast as McDavid and his goal scoring mitts were close to Ovechkin's (or at the worst, Patrik Laine's). He looked like a Hollywood movie star and everything about him was electric. Outside of Lemieux and sometimes Lindros, Jagr and Forsberg, nobody lifted fans out of their seats like Bure in the 90's. He'd challenge for the Rocket Richard annually and likely the Art Ross, based on the way they are letting the talented players perform without obstruction these days.