I agree with those who rated the '76-77 Canadiens over the '84-85 Oilers. The Habs that year were utterly dominant from the beginning of the season to the end of the finals, and no other team even came close.
Iain Fyffe has analysed the regular-season records of every NHL team, taking into account the degree of parity in each season, to compile a list of the most dominant NHL teams of all time. He ranked teams by WPZS – winning percentage z-score.
The top team on his list (which was made in 2002) is the '95-96 Red Wings, with a winning percentage 2.58 standard deviations above that year's average. Since that team did not win the Stanley Cup, it's easy to take it off the list of all-time greats. The #2 team on the list is the '88-89 Flames, with a WPZS of +2.31; followed by the '88-89 Habs at 2.19, another Stanley Cup loser. The '76-77 Canadiens, #4 with +2.18, are the second highest rated Cup winner. The '84-85 Oilers are tied for 42nd on the list at +1.53.
It's true, the Oilers that year had the greatest player in the history of the game, and he was in his prime. But they also had some of the worst players in the league filling out the bottom of the roster, something that was not true of the Habs' dynasty of the 1970s (or the '89 Flames). This does much to explain why they did not have the kind of all-year-long dominance that the Habs used to have.
If we disqualify the Flames as a one-shot wonder, the '76-77 Canadiens easily take the crown.