Which goalie has been the biggest difference maker this year? There are many ways to look at this question, like value over average, team record vs. Pythagorean projection with an average goalie in the net, etc.
However, I am going to look at it in a different way - I am going to compare the records of goalies when they have a quality game (defined as 2 goals against or less) to when they have a poor game (defined as 4 goals against or more), and see which goalies have the greatest discrepancy. That is, which goalies are almost guaranteed to win when they have a good game and almost guaranteed to lose when they have a poor game. The goalies playing for those teams tend to have individual contributions that are very significant in determining the final result.
This categorization is of course very strongly team-influenced. A goalie can be good without being a big difference-maker. It is more difficult for a goalie on a good team to be a huge difference-maker, since the team often makes up for his weak games. Take, for example, J.S. Giguere of the Anaheim Ducks. Giguere has had an excellent season, racking up a .922 save percentage, although he is doing it behind a good defensive team. When he posts a quality game (2 goals against or less), his record is 20-4-3, which is good but not exceptional when compared to other goalies around the league. When Giguere lets in 4 goals or more, the Ducks are an amazing 5-2-2 for a .667 winning percentage. Clearly, the Ducks are an outstanding team and Giguere's performance, while excellent, is not pivotal to their success.
My expectation was that Roberto Luongo would rank very highly based on this measure, since he is an outstanding goalie on a low-scoring team. Others who are similar are Kiprusoff and Brodeur. However, those three goalies all won a reasonable amount of games even when they played poorly. They were certainly large difference-makers, they just weren't the biggest.
The NHL goalie who has been the biggest difference-maker this year to his team has been Andrew Raycroft of the Toronto Maple Leafs. Raycroft has had an up and down season, but his splits are like night and day: when Raycroft has a quality game, the Leafs are 23-0-2. When Raycroft has a poor game, the Leafs are 1-11-2. His play doesn't seem to depend on the number of shots against; Raycroft gets about 29 or 30 shots a game no matter how well or poorly he plays. The correlation coefficient between goals against and shots against for Raycroft is a mere 0.27, indicating very little effect. One reason is that Raycroft has a high tendency to play atrocious games that make it very difficult for his team to win, as he has let in 5 or more goals 8 times in 49 starts and has been pulled 4 times. The Leafs have an excellent record when Raycroft plays well, in part because the Leafs have scored 4 goals 16 times in his 48 starts, seven times winning 4-2 and four times winning 4-1. Part of it is that both goalie and team have been hot at the same times, but the Leafs can score goals so as long as they hold it together on the defensive end they give themselves a good chance to win.
Here is a case where my subjective viewpoint collides head-on with a statistical perspective. My anecdotal viewpoint, before this study, was that Raycroft more or less plays as well as everyone else around him, and people tend to blame or credit him for the result simply because he plays the most important position. However, I think I will have to defer to the stats here. Raycroft was quite a bit more streaky than I would have expected, both in terms of good games and bad games, and it doesn't look like team defence was as big a factor as his personal performance. Therefore, it appears that Raycroft is, like or hate it for Leaf fans, extremely important to the success of the Blue and White.
The second biggest difference maker is Olaf Kolzig. He plays for a mediocre Washington team, and has had an excellent season when that is taken into account, posting a .909 save percentage and helping keep Washington on the fringes of the playoff race despite facing over 33 shots a game. When Kolzig allows 2 goals or less, Washington is 14-1-3; when he lets in 4 or more, the Caps are 1-13-2.
In third spot is Dwayne Roloson of the Edmonton Oilers. Edmonton is 20-3-2 when he puts together a quality game, and 1-12-1 when he plays poorly.
Rounding out the top five are Manny Fernandez and Tim Thomas. Luongo comes in seventh, Brodeur eighth, and Kiprusoff eleventh. Some of the goalies that rank low based on this measure include Miller, Emery, DiPietro, Fleury and Giguere.
I think this brief study supports the belief that goaltending is more important for mediocre and bad teams than for good teams. The teams with the greatest differences were all teams that are struggling to try to make the playoffs. Two of them, Raycroft and Thomas, aren't having particularly good years, but interestingly enough streaky goalies seem to have extra value on bad teams. When they are bad, the team is likely to lose anyway, but when they are good they give the team an excellent chance to win.
Therefore, before rushing to credit goalies on dominant teams for their success, make sure they truly are making a difference in the final results.