Dryden is before my time, but to me he's always the most difficult goalie to 'rate' (not that I spend much time rating goalies... or thinking about goalies).
At least in North American professional + collegiate + NHL hockey, Dryden is surely the "winningest" goalie of all time in terms of wins-losses, no?
ECAC (US College): 76-4-1 (94.4 win%)
AHL RS: 16-7-8 (64.5 win%)
NHL RS: 258-57-74 (75.8 win%)
NHL playoffs: 80-32 (71.4 win%)
His entire career is all-killer, no-filler. The only area in which he wasn't completely win-dominant is international hockey, but even then there's only a tiny sample size of, like, 8 games over three tournaments, and he still went .500.
So, we get into the eternal dilemma of how much of his teams' successes to attribute to him...?
On the one hand, everyone seems to agree that Dryden deserved the 1971 Conn Smythe. Also, as noted, without him in 1973-74, the Habs suddenly dropped 21 points in the standings and went out in the first round (then gained 14 points when he returned the next year). And once he retired in 1979, the Montreal Dynasty was over.
On the other hand, even without him in 1973-74, Montreal was still one of the best teams in the League and had pretty good defence, despite Wayne Thomas (I've never heard of this guy until today) as the main starter. He had 10 NHL games under his belt before that no-Dryden season started, but was the top goalie on the Habs that year, and they had a .635 season, which is still really good. Then, you can also look at the back-up goalies behind Dryden: 'Bunny' Larocque went 96-18-18 during the years he backed-up Dryden. I repeat, the back-up goalie went 96-18-18 (!!!) for five seasons while backing-up Dryden. Now, sure, it was the 1970s and Larocque probably faced a lot of crappy teams, but still that is the most insane record for a back-up I have ever seen in my life. Then, there's the fact that after Dryden retired (and after Bowman left town), the Habs didn't exactly fall apart defensively. In 1979-80, they had the 3rd-best NHL defense, in 1980-81 the #1 NHL defense, and in 1981-82 the #1 NHL defense (#1 of that decade, in fact, with mostly forgettable Wamsley and Herron sharing the Jennings trophy).
So, it's hard to say. But I think the point with Dryden is that even though he was placed into the most favorable position for a goalie (i.e., Montreal anytime in the 60s, 70s, 80s, early-90s), he was the guy who kind of pushed them over the edge, even though he wasn't the straw that stirred the drink.