Overall I'm on the same page as
@Hockey Outsider that Sawchuk as #1 is no longer a defensible ranking, but I will say in Sawchuk's defence that his post-1955 career does tend to get underrated because people aren't properly adjusting for the huge team effects of the Original Six.
First of all, it's a mistake to claim that his peak ended the exact second he left Detroit. He was actually mostly the same goalie in Boston before he got mono:
1955-56 Season:
First 15 GP: 6-4-5, .943, 4 SO
Next 26 GP: 2-20-4, .884, 1 SO
Next 27 GP: 14-9-4, .933, 4 SO
1956-57 Season:
First 26 GP: 15-6-5, .927, 2 SO
Gets mono, misses 2 weeks, returns too early
Next 8 GP: 3-4-1, .897, 0 SO
Announces retirement, leaves team
Sawchuk got a bunch of first-half Hart votes in 1956-57, and they were likely very deserved. If he didn't get sick, he was clearly on pace to add another All-Star level season that year. I also don't know if there was a personal or injury-related reason to explain his dip in the middle of the 1955-56 season, but I wouldn't be at all surprised if that was the case. Overall, if you were watching him play in Boston you probably had like a 75% chance of seeing vintage Sawchuk. There seems to be no question whatsoever that rushing him back into the lineup rather than taking a couple of months to recuperate as most doctors advised would have serious repercussions on the course of his career.
Judging his second Detroit stint is a bit tricky because very few teams in history have ever gone from dynasty to worst team in the league to darkhorse contender over a span of just 7-8 seasons. That 1959 Second Team All-Star selection looks undeserved statistically, but it is perhaps quite understandable given that the narrative of the entire first half of that season was Sawchuk holding his team in despite very limited support. On New Year's Eve 1958, the Red Wings were sitting in 2nd place in the league (
link to the standings), despite being just 5th in team offence with 84 goals in 35 games. After that the wheels came off and the Wings ended up in last, with Sawchuk going 7-21-5, .880 down the stretch, but I certainly wouldn't put all of the blame for that on the goalie.
I think 1960-61 and 1961-62 were most likely Sawchuk at rock bottom, he had personal issues throughout his career but according to reports he really did not deal well with the move to the tandem goalie system, and wasn't even outplaying some relatively mediocre partners. On the other hand, he clearly surpassed his Detroit teammates in the surrounding seasons:
1959-60:
Sawchuk: 24-20-14, .909
Other goalies: 2-9-1, .880
1962-63:
Sawchuk: 21-16-7, .912
Other goalies: 11-9-6, .897
1963-64:
Sawchuk: 25-20-7, .916
Other goalies: 5-9-4, .894
Sawchuk's 1963 2AST doesn't look that undeserved to me, Hall was probably the best goalie and the second best pretty much has to be Worsley or Sawchuk. I also don't think Sawchuk improved in Toronto, he just moved to a better defensive team.
Of course, the flip side of making proper team adjustments for Sawchuk's later career is that you then have to make the corresponding corrections on those dazzling stats from his peak. For example, if we take the average yearly stats from Sawchuk's peak, and compare it to the average of Lumley's two years prior and Hall's two years after, it's not hard to observe the clear positive team effects of the dynasty Wings:
Lumley, 1950 & 51: 34-18-11, 2.38, 7 SO
Sawchuk, 1951-55: 39-16-13, 1.93, 11 SO
Hall, 1956 & 57: 34-22-14, 2.16, 8 SO
(Also keep in mind that the rest of the team was at its best with Sawchuk in net, and also that league average scoring dropped to 2.53 from 1951-55, compared to 2.72 from 1950-51 and 2.61 from 1956-57).