It is always difficult to compare players from different eras .....equipment technology, physical fitness and training ( my freshman year at UMass we had ash trays in the lockeroom ), rules enforcement ( the 1950's-1980's fighting and intimidation era ended right about when Gretzky & Mario rose to the top)
it's all subjective, but here goes my top 5
Howe
Orr
Gretzky
Lemeuix
Patrick Roy
Howe scared the living sh*t out of opponents - he could truly end your career in a fight - and was phenomenally skilled for his era of the straight blade stick - he was a top 10 point scorer for 21 consecutive years !!!!! Many of those were in the top 3 and he led the league in scoring with 95 points in 1953 which was unheard of at that time.
Orr was the complete player - instant acceleration - from 1966-1970 - he had 3 knee surgeries during that period - and a knee surgery back then was a MAJOR ordeal - 8 more during his remaining playing career - Orr was a complete offensive, defensive and a really good fighter and never dialed in a game
Gretzky - played the game with his head not his body - scary - greatest offensive NHL player ever - played hard every shift
Lemeuix - the most physically gifted beside Orr, and probably a smidgen more gifted than Orr, but he took nights off - not just shifts off - but when he really wanted it he was the best player on the planet - period
Patrick Roy - greatest goalie ever - the good NHL goalies make all the saves that should be saved, the great goalies make the saves they should not be able to make, again and again - they get in the heads of the great offensive players - none more than Roy