Roy dominated across scoring levels.
Roy's best and most consistent hockey was circa 1988 to 1992 (throw in 1993 playoffs, though his performance there wasn't quite as great as most people think), when he played behind a defence-first team that had Chelios, Green, Ludwig, Svoboda, Robinson, Schneider, Desjardins, Brisebois, and Lefebvre mannig the blue-line.
I mean, there's a reason Brian Hayward---who was otherwise a forgettable player, with an .843 save percentage the season before joining Montreal---had back-to-back seasons (in '87 and '88) with the third-best save percentage in the NHL.
His 3 Vezinas were in a very high scoring era, as were 2 of his Smythes. His last Smythe was in the middle of the DPE. He had a very strong 2002 and 2003 as well. That he was a top goaltender on multiple teams across 18 seasons in very different scoring environments is a big part of why he is ranked where he is.
"Multiple teams" is kind of flatterring Roy---he played for
two teams, both Cup contenders pretty much consistently during his tenures. Certainly he was a big part of those teams being in Cup contention, but both were also great teams with or without him.
I'm not sure I get your point about "scoring environments" being a point in Roy's favor, though. I would think most people on this forum are aware of the historical differences in scoring environments. Also, what difference does a lower scoring environment make in justifying a higher ranking for a goaltender?? If anything, it's the opposite of what you're saying. Lots of goalies of Roy's vintage (or a little younger than him) look way better than they actually were from the late 1990s onward because of the lower scoring environment.
The fact that a goalie had good numbers, later in his career, in a
lower scoring environment, isn't a point in his favor (in itself).
Brodeur won Vezinas both in the DPE (2003 and 2004) and in the high scoring post lockout (2007).
Brodeur was also great as early as 1993-94, still a quite high scoring environment.
Lemieux was able to dominate PPG in very low scoring years (2001 and 2003).
"Dominate" is over-stating it. In 2002, Lemieux was outscored by his own teammate head-to-head (and only appeared in 43 games total). In 2003, he was second in PPG overall due to a hot first half, and from Dec. 30th onward he was 38th in PPG.
You can make the point that Hasek only dominated in a low scoring era, but I don't think it's very compelling. Scoring was still high when he won his first Vezina in 1994...
Indeed, his 1993-94 is one of the greatest-ever and most dominant seasons by an NHL goalie!
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Anyway, Jacques Plante was the best, and leaves Roy in the dust:
-- first led NHL in save percentage in 1956, last led NHL in 1971
-- led NHL in save percentage five times, including with THREE different clubs
-- highest save-percentage of all time in 1971, aged 42 (despite scoring being way higher than in his younger days)
-- never had a less than .902 season (including playing on a horrible mid-sixties' Rangers club)