Original Six goalies are very difficult to evaluate, because of the extreme team effects. The best talent was usually concentrated on a couple of the best teams, and the goalies on those teams not only had the advantage of playing behind a great team but they also never had to face their own team's elite goal scorers.
It is very difficult to estimate these effects, however, because most teams gave all or nearly all of their minutes to a single starting goalie. Glenn Hall, the most extreme example, played 503 games in a row at one point, so we don't even have a single game's worth of results for any other goalies on his team for a full 7 year stretch. That makes it impossible to use the method of comparing results to backup goalies. There was also less freedom of movement, so goalies didn't change teams as often.
These limitations mean that any method used is going to be less than ideal, but focusing on the goalies who changed teams and trying to estimate the team impact seemed like the best option. I decided to look at all the goalies that changed teams in the post-WWII Original Six era (1946-47 to 1966-67). The goalies were: Glenn Hall, Johnny Bower, Jacques Plante, Gump Worsley, Emile Francis, Hank Bassen, Frank Brimsek, Terry Sawchuk, Al Rollins, Jim Henry, Don Simmons, Bruce Gamble, Cesare Maniago and Harry Lumley.
I figured out the cumulative winning percentage and GAA for each goalie on all of the different teams they played for, and then tried to estimate each team's relative rank based on the differences.
Of course it is unrealistic to assume that every team had the same relative strength over a 20 year period. Montreal and Toronto were consistently good, while Boston and the New York Rangers were pretty mediocre, but Detroit was a powerhouse in the 1950s and terrible in the 1960s, while Chicago was exactly the reverse. Another problem is career arc - someone like Glenn Hall spent his entire prime in one place, and the only points of comparison we have of him somewhere else are either as a very young goalie or as an old one. The final problem was sample size: there were a few team pairings that didn't have a single goalie play on both of them. It was particularly difficult to evaluate Montreal goalies, since there were really only two goalies who played a lot of games in both Montreal as well as somewhere else, and both of them happened to also play for the Rangers (Plante and Worsley). There were also a few well-travelled goalies (like Harry Lumley, who played on 4 out of the 6 teams) that ended up having a larger effect on the sample.
However, despite these limitations, the numbers seemed to validate the method through a reasonable degree of consensus. For example, if you compare the goalies that played in both Boston and Toronto, they had a GAA in Toronto that was 0.42 better and a winning percentage .084 higher compared to Boston. If you used the Chicago results to verify this (by looking at the goalies who played in both Chicago and Boston, and comparing those results to the goalies who played in both Chicago and Toronto), the estimate was that the Leafs were 0.34 better in terms of GAA and .092 in winning percentage. Using the Detroit comparisons, it came out to 0.51 and .037. We can therefore ballpark the expected effect of getting traded from Boston to Toronto as being something like 0.40 - 0.50 in GAA and .070 - .090 in winning percentage.
I took averages from several of these comparisons, and came up with a relative set of rankings:
1. Montreal: 0.00 GAA, 0.000 win %
2. Toronto: 0.00 GAA, -0.040 win %
3. Detroit: +0.15 GAA, -0.025 win %
4. Boston: +0.40 GAA, -0.115 win %
5. Rangers: +0.70 GAA, -0.185 win %
6. Chicago: +1.00 GAA, -0.245 win %
If we compare these numbers to the actual results, we can both verify them and see which teams apparently had strong or weak goaltending:
1. Montreal: 2.36 GAA, .600 win %
2. Toronto: 2.51 GAA, .535 win % (+0.15 GAA, -0.065 win %)
3. Detroit: 2.54 GAA, .561 win % (+0.18 GAA, -0.039 win %)
4. Chicago: 3.04 GAA, .441 win % (+0.68 GAA, -0.159 win %)
5. Rangers: 3.08 GAA, .430 win % (+0.72 GAA, -0.170 win %)
6. Boston: 3.09 GAA, .432 win % (+0.73 GAA, -0.168 win %)
The total results confirm that Montreal, Toronto and Detroit were the three front-runners, with similar GAA totals. Montreal likely did have somewhat better goaltending than the Leafs or Wings, but the main reason the Canadiens had more team success was probably not goaltending but superior offensive play. The model predicts the Rangers quite well relative to the Canadiens, which suggests that despite often being a bottom-feeding team the Rangers got decent performances from the goalie position. Boston, on the other hand, appears to have had weak play in net, since they allowed the most goals of any team but apparently had a better defensive environment than either New York or Chicago.
Chicago's comparative results are exaggeratedly poor because, as previously noted, Glenn Hall was the only guy in their net during the early to mid-1960s. If we take Chicago's results prior to Hall's arrival in 1957-58, the Blackhawks' cumulative goalie stats were 3.46, .345, which means they were 1.10 and .255 worse than the Canadiens, numbers that are very close to my team effect estimate. There seemed to be more goaltender movement in the post-war years than in the early 1960s, which means that these estimates are probably more representative of the league competitive balance in the late 1940s and 1950s, a period where Chicago was consistently the worst team in the league.
These are just ballpark estimates to keep in mind when looking at older goalie statistics. This is also evidence of the dependence of goaltending statistics on team play, since the variance of team effects is much larger than the variance of goaltending play. The model predicts the results reasonably well for 4 out of the 6 teams, as well as for Chicago up until the 1960s. This would most likely not have been the case if there were drastic differences in goalie quality across the league. The difference between, say, a Gump Worsley and a Terry Sawchuk was certainly much, much smaller than the difference between the Red Wings and the Rangers. After looking at these numbers, I'm not sure there was much difference at all between many of the longtime starting goalies of the time period.
This is why evaluating goalies based on on wins and shutouts from that era is pretty pointless - instead of finding the best goalies, you will merely end up finding the goalies that spent the most time playing on the Canadiens, Maple Leafs, or Red Wings.